Melissa,
Pica Pica is not common in the summer. I spend every summer in Belize (also spring and fall-I'm on my way back down this weekend). Pica pica algal and larvae blooms occur in the spring mostly. Usually starts showing up in in March and April and may last through May and sometimes into early Jne.
It does occur at the surface so snorkelers are most affected (I know, I had it really bad a couple of Easters ago while at South Water Caye and a mild case last spring at Glover's Atoll - I was smart then, I got out the water immediately)