This may not work with all the underwater drones on the market, but recently someone shared it on the Chasing Innovation Facebook Group and it worked really well on the Gladius Mini. WD-40 is not to be used on maintenance lubrication of the motors, but in the case of seized up motors from corrosion, it has found to work really well. You can try taking out the motors, spray some WD-40 liberally into them, then softly turn them with a wrench, and that should get them freed up. Worth a try before buying brand new motors for your underwater drone.
Lately I've been seeing this problem, it's telling me that some owners are letting their underwater drones sit for too long without using them or spraying them with silicone spray for maintenance.
Anyways, hope this helps!