I have made a waterproof speaker for my mates RN Oberon class Sub. It consisted of two waterproof mylar speakers, back to back, bonded into a piece of appropriate sized plastic tube and waterproofed fully with silicone sealant. This was designed for a diving sub and to be honest, did not provide a great sound output. Taking into account the relatively small size of the speakers to fit within the tight confines of the model, and the lack of any meaningfully sized holes to let the sound out, it was not very effective. The sound unit fitted was Dive Klaxon from Jotika and sounded pretty cool...just within about 30' of the model!