It is condensation. The cooling passages are open to the combustion chambers via the exhaust system.
You need to change your shutdown procedures. When you pull it out of the water, make sure the front is a bit higher than the rear of the ski, then crank it and rev it about 3 or 4 times. Don't run any more than 5 to 10 seconds (15 by the book, but 5 to 10 is plenty). Then pull the black knob on the big exhaust pipe and crank over again about 2 or 3 seconds. That should blow all the standing water out of the exhaust. This should also make the ski easier to start next time out.
