I am fitting a Pioneer to my RIB at the moment. However I have a small access door in the back of my console with the VHF in which my stereo also fits in
so is away from nasty water. For speakers you can get water proof ones at stupid money, or Kenwwod do some speakers with no metal parts to rust and look horrible, and now most speaker cones are not made from paper so do not fall apart.
With regard to fitting your stereo, depending on where you want to mount it, splash proof covers are available, as you would find in most sportboats now.