Fantasy Gone Awry or Instinctual Warning?

Most humans would agree that this is (at least) attractive:

Judging the photo merely on objective grounds, one identifies the doe-y, come-hither eyes; the slightly open mouth; blonde, somewhat tussled hair - dry, even though she's in a damn swimming pool. How about that swimming pool? Isn't part of the allure the fact that she's in water? Isn't there something inherently sexy about scantily clad women in water?

Is that why so many mythologies have these:

Artist Unknown

Yikes. 
The Rusalki (sing. Rusalka), "Red Haired Girls (русалка)", of Slavic mythology are female water nymphs who are believed to be confined to pools and founts except during a week in June, Rusalka Week/Green Week (русальная неделя), during which the nymphs were especially dangerous and they could escape their waters; swimming was forbidden during the week. It was the custom of the Rusalki to lure children and young men to swim with them in the water, only to drag them to their deaths or tickle them to death under the waves.
Holy shit.

What about these lovely ladies?

Hylas and the Nymphs, Waterhouse

They seem so nice and...interested. Like...really interested. And then once you're in the water (which is really good, they're telling you), they have you.
This happens:

αἶψα δ᾽ ὅγε κρήνην μετεκίαθεν ἣν καλέουσιν

        And forthwith he (Hylas) a spring followed after, which they call                       

Πηγὰς ἀγχίγυοι περιναιέται. οἱ δέ που ἄρτι

                  The Pegas by the neighbors who dwell around. And there, in that place

νυμφάων ἵσταντο χοροί· μέλε γάρ σφισι πάσαις

                  The Nymphs' dances are held. For it was in their care, all of them

ὅσσαι κεῖν᾽ ἐρατὸν νύμφαι ῥίον ἀμφενέμοντο        1.225 A

                  Who there in the lovely headland, the nymphs who there dwelt,

Ἄρτεμιν ἐννυχίῃσιν ἀεὶ μέλπεσθαι ἀοιδαῖς.

                  To Artemis in nightly songs ever to sing.

αἱ μέν, ὅσαι σκοπιὰς ὀρέων λάχον ἢ καὶ ἐναύλους

                  They either the peaks of the mountains had by lot or even the streambeds

αἵ γε μὲν ὑλήωροι, ἀπόπροθεν ἐστιχόωντο·

                  But also the wood-watchers, far-ranged they wandered.

ἡ δὲ νέον κρήνης ἀνεδύετο καλλινάοιο

                  But one, from the top of the beautifully-flowing spring rising 

νύμφη ἐφυδατίη. τὸν δὲ σχεδὸν εἰσενόησεν        1.230 A

                  Was a water-nymph. And him she nearby perceived

κάλλεϊ καὶ γλυκερῇσιν ἐρευθόμενον χαρίτεσσιν,

                  Him, with beauty and with sweet graces blushing red,

πρὸς γάρ οἱ διχόμηνις ἀπ᾽ αἰθέρος αὐγάζουσα

                  For forth lighted by the moon from the upper-heaven, fully brighted,

βάλλε σεληναίη· τῆς δὲ φρένας ἐπτοίησεν

                  Him the moon smote. And her heart did she make faint, 

Κύπρις, ἀμηχανίῃ δὲ μόλις συναγείρατο θυμόν.

                  She Kypris, and in the confusion only just did she gather back her senses.

αὐτὰρ ὅγ᾽ ὡς τὰ πρῶτα ῥόῳ ἔνι κάλπιν ἔρεισε        1.235 A

                  But at the first into the stream the pitcher he dipped

λέχρις ἐπιχριμφθείς, περὶ δ᾽ ἄσπετον ἔβραχεν ὕδωρ

                  To one side leaning, and wordless ringing the water 

χαλκὸν ἐς ἠχήεντα φορεύμενον, αὐτίκα δ᾽ ἥγε

                  Into the bronze repeatedly ringing, and immediately she

λαιὸν μὲν καθύπερθεν ἐπ᾽ αὐχένος ἄνθετο πῆχυν,

                  On his left, from above his throat she laid her forearm,

κύσσαι ἐπιθύουσα τέρεν στόμα, δεξιτερῇ δὲ

                  To kiss, yearning for his soft mouth, and with her right hand 

ἀγκῶν᾽ ἔσπασε χειρί· μέσῃ δ᾽ ἐνὶ κάββαλε δίνῃ.        1.240 A

       His elbow she drew down and cast him into the middle of the eddy.

-Apollonios of Rhodes, Argonautika A.1207-1240, interlinear with my own -ANF.

The Grecian Ναϊάδες (Naiades) - were freshwater nymphs (saltwater nymphs being Νηρηΐδες [Nereides - "The Daughters of Nereus", the Old Man in the Sea]) and tended to be bound to certain founts and waterways (as the Rusalki). The Naiades very famously seduced Ὕλας (Hylas), a young Argonaut and companion of Ἡρακλῆς (Herakles), whose beauty fascinated them, into entering their pool where they drowned him (Apollonios has one nymph seducing the youth, other authors, such as Strabo, have several). 

The tripartite Rhinemaidens, examples of the Germanic nix or nixie, are water sprites.

The Rhinemaidens Warn Siegfried, Arthur Rackham

Though most famously found in Wagner, nixie do show up in Das Nibelungenlied as well, though in connection with the Danube. Here, the sneaky Burgundian knight Hagen gets the better of two rivermaids:

1533 [Hagen] sought to find the boatmen / if any might be near,
When sound of falling waters / full soon upon his ear.
Beside a rippling fountain, / where ran the waters cool,
A group of wise mermaidens / did bathe themselves within the pool.
1534 Ware of them soon was Hagen / and stole in secret near,
But fast away they hurried / when they the sound did hear.
That they at all escaped him, / filled they were with glee.
The knight did take their clothing, / yet wrought none other injury.
1535 Then spake the one mermaiden, / Hadburg that hight:
"Hagen, knight full noble, / tell will we thee aright,
An wilt thou, valiant warrior, / our garments but give o'er,
What fortune may this journey / to Hunland have for thee in store."
1536 They hovered there before him / like birds above the flood,
Wherefore did think the warrior / that tell strange things they could,
And all the more believed he / what they did feign to say,
As to his eager question / in ready manner answered they.

 -Das Nibelungenlied, Author Unknown, trans. by George Henry Needler

Mermaids?
 

A Mermaid, Waterhouse. Aww, she seems nice. She has a tail.

"Seamaids" from the Old English mere "sea" and maid, these creatures appear in mythologies the world over, from the Greeks to the Chinese:

Greek legend changed Alexander the Great's sister Thessalonike into a mermaid who haunts the waters of the Aegean. She sets upon a ship in a maelstrom and asks only one question: "Ζει ο Βασιλεύς Αλέξανδρος;" - "Lives the King Alexander?"; to which there is only one answer: 
"Ζει και βασιλεύει και τον κόσμον κυριεύει" - "Lives he and even reigns and over the kosmos is the lord." Pleased with this answer, she quiets the maelstrom; any other answer will cause her to dash the ship to bits.
In Chinese lore, sailors take home mermaids stranded on the shore. Once they released, they express gratitude. These creatures, which are either covered in fine, multi-colored hair or have webbed hands and feet, seem far more kind than their Western counterparts:

The Fisherman and the Syren, by Frederic Leighton c. 1858

This guy looks like he's having fun. I'm sure he notices her tail wrapping around his legs. Is that water level rising?

Just so we're clear: if one should find oneself near a pond or other body of water and an attractive woman (or maybe man, who knows?) invites one into the water - take a deep breath and consider that maybe most of these stories are similar for a reason:

The Siren, Waterhouse. I'm not convinced that this guy is having fun.

So don't be this guy - maybe skip out on the babes inviting you into the water.

Sirens
How did Sirens and mermaids become "intertwined", pun intended?
Σειρήν (pl. Σειρῆνες) "Seiren/Seirenes, commonly Anglicized "Siren" - "The Entwine-ers" (get it?), were originally the handmaidens of Demeter. When Persephone was kidnapped by Haides, Demeter gave these handmaidens the wings of birds in order for them cover more ground with greater speed. They grew weary of their searching and so escaped their quest to the island of Anthamoessa, where they become accustomed to using their bewitching songs to ensnare passing sailors into dashing their ships into the rocks and cliffs nearby.

Ulysses and the Sirens, Waterhouse

When the Argonauts passed by their isle, the musician Orpheus played his music so loudly and beautifully that it drowned out their songs.
When Odysseus and his men were approaching them, the wily king was so overcome with a desire to hear them that he ordered his men to lash him to the mast (above) while they stuffed their ears with cloth and wax - he could not resist from hearing their song. Pure temptation itself.

Over time, the legends (or facts, if you will) of the mermaids and sirens became fused.
Intertwined, right? 
Here's another "Odyssey and the Sirens". Compare with the above:

Ulysses and the Sirens, Herbert James Drapper

Slightly more fishy, less wingy.