You can use a pump same as a return pump for a
SW tank. All you need is a float, emergency float/alarm, check valve, etc. Google ejector pump installation, diagram, plumbing, etc. and you will see the design. Then just use an aquarium pump if you are worried about the
SW rusting it out. There probably are pimps rated for marine environments that can pump massive amounts but sounds like all you need is to remove
PWC amounts.