For good eats in a fantastic natural riverside setting, take a taxi down to this restaurant at the El Gran Mestizo on the banks of the New River south of town. For the full experience pick a table in one of the waterside palapas (open-air shelters with a thatched roof) and choose from Belizean, Mexican, seafood and international dishes on the ample menu.
There's also a full bar and cocktail list; bring bug spray if you want to dine outside.