Here are three, in the order that I remember liking them (it's been a year):
Wendy's
Cozy Corner
Daisy's
As for ambience, most Placencia restaurants had less of it--there were very few oceanfront restaurants (I ate at one or two the whole week). As for something nicer, you could go to Turtle Inn up the road. It's definitely got ambience, though I've never eaten the food there.