Search for epoxy coated rare earth.
Some of the best prices I’ve found are here: http://www.magnet4less.com/
I get the black epoxy coated one, then encase them in PETG for extra protection from scratches, as the epoxy can come off if you let them bang together.
I don’t think they exist. I have had encased magnets from just about every marine aquarium vender fail on me over the years. Some in a very short time, others it took years of use. Any magnet submerged in salt water should be checked at least yearly for signs of corrosion and disguarded if it showed any traces of rust. Swelling around a magnet is a sign the magnet inside is corroding and should also be removed.