Vulgus is a little on the non-consensual side compared to most authors on SOL. There are a few, like Switch Blayde and Linda Jean, that regularly write in the some of the darker themes, but most here only dabble in them occasionally.
Since you're asking on the forum, I'd wager you aren't yet a premium member. I'm not sure if Category search is available to you. If it is, you can search by category code for what content you want and even filter by length of story, score, etc. Blackmail, watersports, beastiality and prostutution are all existing codes. As a premium menber you can even search for keywords in the description or even the story text with advanced search. That might be the path you want to take if you can afford it, as your likes are fairly specific.
If you use the basic title search box on the homepage and simply put in "Blackmail" or "Dog" you will get a decent number of returns with that word in the title that will match your inclinations. Those are the authors likely to be writing the kind of thing you want.
I'm not a watersports fan so I can't help with recommendations for that specific fetish. Rachael Ross is the preeminent beastiality writer on the internet.
Powerone probably has the work closest to Vulgus in hard non-consensual content here. Lord John Thomas has a couple of pretty decent yarns about blackmailed and coerced wives on SOL. Charm Brights has a lot of forced-into-sex stories most of which are sci-fi or harem based. Not My Story has a few as well, for the closest thing to what you asked for check out his "Everybody Loves Raymond's Wife".
John Argus is a hugely prolific porn paperback author and probably the writer that most closely matches what you are looking for. Most of his stuff you will have to pay for but it's available in electronic form several places. He also published as Dean King for his really hard-core stuff. You can find some of his earlier work online as "Dark Dreamer" Lia Anderson is another good dead tree porn writer. I think her "Degredation of Tricia" is available online for free in a few places.
There are a good number of stories on Djian's Understories site you would probably like, broken down by category. For watersports you'll probably have better luck on BDSM Library. ASSTR is, of course, the motherload of the weird and transgressive but finding stuff there is kind of hit and miss. If you check the collections there you will find some good stuff. That's where I first ran across Vulgus.