Let's see: boats are almost always used near water....
Ever get into a water fight with a rival boat?
Ever have a rival rider come cruising near by, and throw a sheet of water over the whole boat?
Ever get caught in a rain?
The speakers don't need to be "waterproof" but they do need to be moisture resistant. Audio purists may prefer the sound quality of a paper cone but the paper will simply absorb too much moisture. Cones made of a plastic material will give you much better longevity.