If you use waterproof speakers, such as Poly-Planar, I do not think it would be a problem.
If you are using a regular "Marine Grade Speaker" check that they are waterproof and not just marine grade. Marine grade does not mean waterproof.
Poly-Planar speakers can be submerged in water without damage and they have a very good sound. Not as good as a high-end MB Quartz, JL Audio, or some other automotive speaker, but these would not last long if they got very wet.