Some very generous folks here, which is good!
However, I don't think 20-25% is typical of what the average tip is.
With some exceptions, I suspect that Belizeans tip somewhere from a few loose coins to 10%.
Most Belize restaurants, such as those at fancy lodges and hotels, that include a service charge in the bill, are typically at 10%. Even Cayo Espanto, the most expensive resort in Belize, levies only a 15% service charge.
San Pedro probably has higher tip levels than most of the rest of the country, since the tourism culture is more engrained on Ambergris Caye.