Do go into the water! Original post here.
Bonus Image Provenance: I commissioned Bacchus at ErosBlog to research this image further, and he has come up with the following additional details.
This image stream on Tumblr originated here:
Surprisingly for Tumblr, at that location there is a rare “source” attribution to this URL:
That .png is 20 pixels wider than all the tumblr versions, and is the highest-resolution image on the net. With a little exploring of directories, we learn that it’s at the “Monster-Girls Uploader” site:
It’s actually linked from this page, where we can see it was uploaded in 2007:
It’s identified (captioned?) there with these characters: [Editor’s note: unfortunately the characters didn’t survive transmission]
Google translate identifies this as the Japanese word for Dagon, which I’m taking for the image title (lacking any other candidates). The English-language Wikpedia entry for “Dagon” identifies an ancient (~2500 BC) Mesopatomian god of both fish and fertility, sometimes drawn as a mer-man in more modern sources:
The autotranslated Japanese wikipedia entry for Dagon offers us the additional tidbit — likely relevant to this image — that Dagon features in the Cthulu mythos as the founder of a half-fish, half-human deep sea cult. It’s no great leap to imagine that your friendly Dagon-girl is recruiting!
Searching further, I discover your image on Danbooru (uploaded in 2008 or prior) with a tag for the artist name of “ze (sawakihein)”:
That tag is a hyperlink that takes us to several pages of additional artwork with the same nametag:
Notable and potentially of interest among these is a frog girl and tadpole sidekick:
I was unable to develop any English-language information about this artist, but I was able to turn up two similar galleries that substantially overlap the content for this artist at Danbooru:
Bacchus is actively taking image research commissions, and if you have adult imagery you’re curious to learn more about, I encourage you to visit Bacchus’s introductory post for his image-searching service, where you can find details about how to commission him.