I saw a Gladius Mini owner talk about using Q8 silicone spray to extend the live of the motors on your underwater drone, basically to protect the bearings from corrosion. However, there's a downside as he mentioned this could void your warranty. Someone also mentioned Aqua Stop as a good alternative too for electronics, motors, and bearings.
A big problem with silicone spray is, it can get toxic to aquatic life with long lasting effects.
Hopefully this thread will get updated a lot, as I'd like to use something that doesn't avoid my warranty and is safe for marine life. Best thing you can do for now, is always run your underwater drone with some clean water in the tub.