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.