Yes,they can stay underwater.Tunze turbelle 6040,6055,6095,6105 models use magnet holder with silence clamp up to a glass thickness of 15 mm (1/2").
Both halves of the magnet holder are waterproof.:bounce2:
If your magnets are unsealed,then they are are ferrite,the same ceramic magnet material that impellers are made from.These magnets will not corrode.Only the neodymium magnets need to be epoxy sealed.