15k population? 30% Expat? Wild guess and also not sure on your definition of expat. If you mean Whites it's prob <5%. Belize is a huge melting pot of cultures, nationals and races.
The population also varies quite a bit between high and low season as many will go back to their home town/village on the mainland in slow season.
Population census' and Electoral Registers show San Pedro @4k as most are registered in their family home town/village, so no accurate records.