Hard to do and you can't get a permit easily on a vacation. Done discreetly no problem. Asking around in Placencia,and talking about your idea, it may not work out. Can't take a cooler of lobsters home to US without an export license either,no matter who caught them You can get one in Belmopan fairly easily.
Placencia is not very good snorkeling from shore. The beaches are beautiful but the turtle grass starts about 10 ft out so swimming is not that good unless you go to the town where the boats anchor, its sort of a split but all sand bottom. The reef is 16 miles out. Their are some spots closer but you still need a boat, and a licensed Captain and guide.