When i was a kid, my dad and i watched a movie about a space ship that landed underwater next to a seabed lab. The alien hatched out of....a hotdog in an ice cube.Well, triangular, not a cube, as such. In the words of Dave Barry, "I am not makeing this up." The alien fish monster thingie had to be fought off, and i think the hu-mons won. Anyone else see this thing? And is it available?
Now, whether or not this is the film you're thinking of or not, I don't know. I haven't seen any film like it. However, reading your description of the film reminded me of a review I read on the "And You Call Yourself a Scientist!" site. The film was called "Destination Inner Space" there, but it is also called "Terror of the Deep".
The DIS review is at http://www.penrithcity.nsw.gov.au/usrpages/rick/liz_dis.htm.
The DIS IMDb entry is http://us.imdb.com/Title?0060302.
Hope that helps.