Mata Chica on North Ambergris is the closest thing to a spa on the island.
I'm not a spa guy, but from what I've seen I'd say the best spa in Belize is at Chaa Creek in Cayo. Beautiful facility, professional staff.
Maruba near Altun Ha appeals to some.
Royal Mayan, sorta of a spa, in Benque Viejo has closed.
Soulshine in Placencia says it is a spa. To me, it's not.
A couple of other hotels such as Inn at Robert's Grove have a few spa-like services but aren't really spas.