Both are waterfront, but not the same product
La Paz is a desert-and-sea market with Mexican infrastructure, Spanish-language systems, and Baja logistics. Belize is tropical Caribbean, smaller, English-speaking, and oriented around island, beach, lagoon, and jungle property.
Ownership friction
La Paz sits in Mexico's restricted zone, so foreign residential buyers generally use a fideicomiso trust. Belize usually allows direct fee-simple ownership by foreigners. That is the cleanest Belize advantage.
Belize alternatives to compare
For waterfront and boating, compare San Pedro, North Ambergris Caye, beachfront land in Belize, and Placencia.