I would not disassemble the gearcase unless the gear lube shows signs of water/milky when you drain it….I would pressure test the unit prior to disassembly if the lube shows signs of water, you want to be sure you know just where the actual leak is before resealing the unit. The impeller housing will need to be screwed down in place if you decide to pressure test, or the pressure will just push the upper bushing seal off its gasket creating a big leak!