I've been told that the water in Placencia is the safest water to drink in all of Central America. I believe that, too. I personally think it is better than the water in the States where I'm from, Berlin, Maryland. You can taste the chemicals in the water where I'm from. The water here tastes like JUST water. I cannot taste anything that may have been added.
That being said there is store bought reverse osmosis water available in all the stores. A 5 gal. bottle cost about $6 bz or $3 us. That is the highest quality you can get and it's affordable.
(Belize River, south of Placencia, Belize, pictured.)