I think you're better off not making any assumptions. I know here in the Rocky Mountain West, I've heard it both boy and boy-ee, as well as boo-ee, and even if someone does say boi for a juvenile male it's no guarantee they will call a water marker the same thing.

My Rog, from New Hampshire, says boo-ee, but then he burr-ees things, too and that's one I'd never heard when growing up. We always berry-eed things in the ground.