The magnetism of the speaker will have zero effect on radio signals. However if the metal of the speaker is in the way of the radio signal the signal might get blocked at certain angles – 2.4GHz is very susceptible to this kind of thing being a very short wavelength, having a very short aerial which can easily be hidden.

Too close to a relay the stray magnetic field "might" upset the relay release ability since relays do use magnetism to operate and hold, but this is very unlikely.

What Dave said about mounting the speaker – there should be no direct air path between front and rear and there should be a clear path to the outside world else you won't hear it with the boats lid on. To keep water away from a paper cone, take a look at booking office/cashier technology, have a sheet of very thin polythene between the speaker and the rest of the world. Left loose, it is totally transparent to sound, but stops water.