I gave up using a wet vac on my cruiser stern. It is designed to have a wet bilge, and and takes multiple empties with a vac.
It does have an automatic bilge pump, but this still leaves about 2" of water.
So now I also use a cheapie small bilge pump fixed to the end of a stick, with crocodile clips to temporarily attach to the domestic batteries. I also use a longish piece of hose, which will reach the far side of the towpath. So I just dip the pump into the bilge and it runs continuously, with no emptying of a vac., until I'm happy with the result. If there is a bit of oil as well, it doesn't much matter as its not going into the cut. I find this most useful for emptying the bulk of the engine tray especially, as this cannot be dealt with automatically.