I have no experience with the PolyPlanar brand, so take this with a grain of salt.
Speakers work by having a fixed magnetic field created by a large magnet acting against a small coil of wire driven by the amplifier. The small coil is attached to the speaker cone. Current flowing through the small coil moves the cone and produces the sound.
Low magnetic field? Seems it would be a good thing. I've heard of numerous folks having the autopilots messed up by the speaker magnetic field. Downside to low magnetic field would seem to be lower sound output. Reducing the fixed magnetic field will reduce the force on the speaker cone for a given driving current.
Waterproofing? Most speakers have the moving cone made of a cardboard material. Water has an obvious bad effect - turning the cone and its support ring into cream cheese. Question would be: what is the 'waterproof' speaker using for materials to make it waterproof?
Speaker cones tend to be light (hence flimsy) to give the most motion for a given 'oomph' from the coil, and hence the most sound. Resistance to breaking seas is not a speaker design criteria. I would recommend storm covers if the speaker cone is something keeping the water out of the people tank.