Yeah, you have to tell us exactly what kind of riding you are doing. If you are just casually riding on the water, no subs, no stabs, etc. then you may have a leak inside-one of your cooling lines may not be completely secured or have a pinhole leak in it- put the ski on the trailer and run it in a lake to see if you can identify the leak.
If you are going crazy, subbing and all the other underwater antics, like the others say, put a bilge in. You actually should always have a bilge in regardless. As for the duckbill...no way. Instead buy a versiplug.
http://www.shop.pwcdoctor.com/main.sc;jsessionid=B4AA5F9A17C41577ADF68A88CFA65C72.qscstrfrnt04
I used to run duckbills, but after one fell off and my ski sunk, I said never again. I have been running the versi for about 2.5 years, not one incident. Just make sure you clean your engine compartment of zip tie ends and other debris, as these have a tendency to make their way to the back of the ski and can keep the versiplug open and thus swamp your ski.
With the bilge and the plug, my ski is BONE dry, no matter where I ride or what I do.