I've also talked recently about being on a Lovecraft bender, searching the internet for public-domain stories from his predecessors and contemporaries. I won't weigh you down with a long list of names or links, but I will say that Robert Bloch is by far the best writer in the crowd. (As an aside, I'll also say that, having seen and read lots of Weird Tales stories by now, I'm amazed at what those 30's-era authors and artists got away with. Naked women left and right, just barely disguised sexual imagery in every climax, and gore enough to make Stephen King applaud.)
Well, I wasn't kidding about the vampire mermaids. I've just finished reading a story by Robert Bloch about a sorceress who inhabits the body of an alluring sea-creature, then saps a male victim of his will with repeated dreams and steamy hot kisses (which were no doubt more than kisses, nudge nudge wink wink) so that she can...well, I'll keep that a secret in hopes you'll read the story. I've uploaded the PDF here.
And just to whet your appetites, I'll share this little excerpt where she lures him into a clearly vaginal cave (Notice how he has to squeeze between two jutting points of rock) and sucks a little more of his will away:
The salt breeze blew coldly across his face, and the sea glistened like silver in the moonlight. A faint splash told of a sea-thing that broke the surface of the waters. To the north the sea washed against the rugged surface of the cliff, barred and speckled with black shadows. Dean felt a sudden, inexplicable impulse to move in that direction. He yielded.
As he clambered over the rocks he was suddenly conscious of a strange sensation, as though keen eyes were focused upon him—eyes that watched and warned! Vaguely in his mind rose up the gaunt face of his uncle, Michael Leigh, the deep-set eyes glowing. But swiftly this was gone, and he found himself before a deeper niche of blackness in the cliff face. Into it he knew he must go. He squeezed himself between two jutting points of rock, and found himself in utter, dismal darkness. Yet somehow he was conscious that he was in a cave, and he could hear water lapping nearby. ... He stepped forward, and, as the floor shelved sharply downward, stumbled and fell headlong into icy, shallow water. He felt, rather than saw, a flicker of swift movement, and then abruptly hot lips were pressed against his.
Human lips, Dean thought, at first.
He lay on his side in the chill water, his lips against those responsive ones. He could see nothing, for all was lost in the blackness of the cave. The unearthly lure of those invisible lips thrilled through him.
He responded to them, pressed them fiercely, gave them what they were avidly seeking. The unseen waters crawled against the rocks, whispering warning.
And in that kiss strangeness flooded him. He felt a shock and a tingling go through him, and then a thrill of sudden ecstasy....
Again, if you'd like to read the full story - which is well worth your time - just click here.