1980's Humanoids from the Deep (also known as Monster) is sort of a weird movie. It's basically a marriage between the goofy plots of 1950s B-movies and the excessive sex and violence that permeated the horror genre during the 1980s. I've always felt like it worked though, but then again movies about sea monsters/sharks/etc. have always creeped me out ever since I first saw the original Jaws. I guess it has something to do with how alien an environment the ocean is for us humans or something. I don't know. As far as the movie goes, the special effects aren't exactly impressive or anything, but its a fun little piece of inconsequential horror.
I can't really comment on the DVD release since its long out of print and goes for a premium on eBay, but it seems to have contained some interesting bonus features, most notably featurette wherein film critic Leonard Maltin interviews legendary B-movie auteur Roger Corman about his involvement with the film.
Corman ordered that additional scenes be shot after the movie wrapped because he felt that there weren't enough scenes of sex and violence.