You probably know that El Pescador is the hotel that really specializes in fishing on Ambergris. However, you don't have to stay anywhere specific to fish, as you can hire a guide regardless of where you are staying. I'd suggest looking back through this board for the many, many reports on various hotels on the island. You might also check my Free, Fun and Easy Guide to Ambergris Caye on our Web site (which rates the various hotels on the island candidly), or buy the most recent edition of Fodor's Belize & Guatemala Guide, which I updated.
Among the many good places I can recommend for quality and fair value are Banana Beach, Villas at Banyan Bay, Coconuts, Paradise Villas, Caribbean Villas, Mayan Princess, Belizean Reef Suites, The Palms, and there are many others.
Belize First Magazine http://www.turq.com/belizefirst/