Many bars have regular Belikin on tap. The treat is when a bar has Stout or Light House or both on tap. I enjoy regular draft at Island Perk, Playa (they have Stout too), Fidos, Bayside, Tackle Box. Riverside Tavern in Belize City and the Barrier Reef Sports Bar in Caye Caulker are the only two bars I know of that have all three beer regularly (on tap). Bottles of regular beer are available everywhere.
Beer is way more expensive per drink than local rum - most of the happy hour specials are based on local rum rather than beer.