I can't imagine that's a good idea. The Bimini top I had certainly wouldn't hold any extra weight. Even if it did, the extra weight up high would certainly make the thing shake and sway a lot at high speeds.
Had a friend mount two on the back rail of the toon so he had tunes when swimming behind the boat in the cove. Depending on what type of boat would depend where you put them. I agree Bimini tops aren't really strong enough to hold cans like that. You could always just take the speakers out and mount them in the seats or console. Maybe even come up with a clamp for front rails or screwed into the side of the boat somewhere.
