Lots of good ones, of different types and at a wide range of prices. Truly excellent, and priced accordingly, is "Red Ginger" at the new Phoenix resort. At the opposite end of the strip is Blue Water Grill at Sunbreeze Hotel, under the same ownership (they also own a third much cheaper restaurant, Caliente between those two, but IMO it isn't as good as it used to be). Not far from Blue Water Grill is Wild Mangos, a boutique restaurant with superb reviews. Moderate-high pricing. On Middle Street you'll find Reef Restaurant offering mainly fish dishes, superb and low prices, and almost opposite it one of San Pedro's best ice cream parlours DandE's (its main competitor is Manelli's in the same position on Front Street). On Front Street towards the southern end is Crave Restaurant, a far-too-little known gem. Excellent food imaginatively presented at very good prices - they do superb lunch specials. There is a Cuban restaurant on Front Street, but in all honesty I can't recommend it - the last meal I had there was low quality surlily presented at astronomical prices. Near the top (northern) end of Front Street is a bar (Lola's) which houses a small restaurant Banh Mi which does superb Thai curry and other Thia and Vietnamese dishes at moderate prices (I must get back there for another "fix" of Thai chicken curry!). On the beach near the middle of Front Street and just a short walk from your hotel is the ultimate breakfast/lunch bar - Estele's. For a light breakfast or lunch you may also like Mathieu's Deli, well up on Middle Street near the roundabout.
Sorry if I've forgotten any, as I surely have. One point to bear in mind. The legal currency of Belize is the Belizean $, fixed at US$0:50. Because of the fixed and straightforward relationship, US$ can universally be used in place of Bz$ (at 1:2 of course). It is not illegal for prices to be quoted in US$ and pretty well all tours and dives (and real estate) are so priced, but it's unusual for restaurants to charge other than in Bz$. I would be VERY wary of one that charges in US$. But that's not all. For some years now it's been ILLEGAL for prices to be quoted exclusive of sales tax (called GST locally). That applies to restaurants, bars, hotels, shops - everywhere. That doesn't stop some unscrupulous individuals from adding GST to the bill when it comes to payment. Remember - that is neither normal nor is it legal. My strong recommendation if you encounter such an establishment is to walk away and go to the next one.
Similarly, too many hotels add a 10% "service charge" on top of the 9% hotel tax. The latter will often be added afterwards, but my recommendation is to patronise establishments that are honest and tell you the full price up-front - ie. including hotel tax (which is a different rate from GST). As to the "service charge" - when have you ever stayed at an hotel that didn't need to provide service in order just to function? Sadly it is VERY common to add this charge and AFAIK it isn't illegal, but I think it IS dishonest and I would avoid places that add it.
The only credit cards likely to be accepted are Visa and Master. Accepting these cards is very costly for the business (for technical reasons I won't go into unless you ask) so expect to pay top rates when you use cards. Some really good establishments don't accept cards at all - Estele's is a notable example. By all means bring your card, but I suggest you bring a decent amount of US$ cash and some low denomination US$ travellers' cheques. With cash & travellers' cheques you will usually be able to negotiate reduced rates at say tour companies and dive centres.
When you book tours/diving through an intermediary such as your hotel you will be paying an additional amount as commission for the agent, usually around 1/6 of the amount you pay. If you know who you want to go with you'll usually save a considerable amount by going straight there and booking directly.
Can't think of any more - I apologise for anything I've missed.