You CAN get speakers which are specially-shielded to reduce their magnetic field to almost nothing, allowing them to be mounted almost anywhere. They use additional internal magnets to help cancel out the magnetic field from the speaker magnet.
One popular make of shielded marine speakers is Poly-Planar. Their 4000 series has 4”, 5” and 6” round speakers which handle 80watts and are waterproof (plastic frames, plastic cones, plastic tweeters, stainless steel mounts). West Marine sell the 5” type for $65 a pair.