That's fairly low compression. But it's even. Another important test to do is a leakage test. It's more involved than a compression test, however if your motor isn't air tight it's not going to run right, especially with the added resistance of being in water.
How well does it run out of water? Are your carbs tuned? Spark plugs wet or chalky white? How long do you have to crank it for it to start out of water?
There's no pump inside the tank, the fuel pump is integrated into your carburetor. The tank is pressurized with two check valves, they're probably up front under the front storage compartment.
Also, food for thought, if you're cranking and cranking until your battery dies, you're doing a lot of harm to your starter , solenoid, and to the battery. If it requires significant crankage to start your ski, something else is maladjusted (carbs, tired starter, engine leakage, low compression, etc.) that could leave you stranded out in the water later on
