Top of my list is a little piece of cinematic trash called Shockwaves.

It's basically an Aquatic Nazi Zombie movie. I think they had a good idea and had the bones of a decent story. But alas they apparently had the kind of budget that was doubled by the producer chipping in all the spare change in his ashtray and they blew most of the budget they had on about 30 seconds of Peter Cushing in the movie. I think with a real budget and actors who were (with the brief exception of Peter Cushing) not rejects from local community theatre, it could have been a decent horror film instead of the slightly snoozy little film it ended up being.
Any movies anyone else would remake if they could?