The premise sounds interesting and the movie poster is fantastic. Too bad it doesn't deliver.
When it comes to effects like you mention, it's very difficult to use actual footage and make it effective. In a low budget monster flick, you can laugh at the effects, but this sounds like a more serious movie. I think I'd rather they use practical effects and makeup instead. Like you said, Wild Kingdom was on every week and was very popular.
While I do like Ernest Borgnine, I think I would skip this one, unless I went on a 70s sci-fi kick.
The movie poster was what convinced me it was worth watching. Good job, ad team.
I think even if they had rear projection footage of fish with scuba guys swimming in front it would have been something. There were very few effects or conflicts to overcome. The fish, crabs, etc mostly just swam past them. Cheesy effects would be better than no effects.