You mentioned that the all the oil drained out, did it all end up at the bottom of the bilge? You also mentioned that you found a crack in the oil fill tube, is this where the oil is leaking out? Have you determined how the oil is leaving the engine?
As bmann stated, check the coolant to determine if any oil has contaminated it and to check for proper mix and protection level. If you determine that the coolant system is intact, then you need to figure out how the oil is getting out of the engine, if you have not already. I would clean up whatever oil is at the bottom of the ski, add the correct level of oil to the engine and let it sit overnight. If the oil level stayed steady and didn't leak out on its own, then I would start the engine on the trailer connected to the hose for a couple minutes and thoroughly observe in and around the engine. You should see where it is leaking from. With that information we can start to determine how to fix it. It could be something really simple such as a hose connection, but also something more serious.
If you dropped the ski off to your dealer end of last season in fully working condition, and it came back to you this spring with these problems I would certainly be skeptical of their work and the fault may rest with them. It may be worth a visit to their shop to discuss the issue with them, they may be willing to diagnose the problem for you (free) and then you can go from there. If a shop did this to my boat I would be hesitant to take it back to them to fix it. It really all depends on your level of mechanical know how with this ski, if you are comfortable diagnosing this yourself, all the better. If not, find someone you can trust to diagnose this for you and go from there.
Good luck, sure not the way any of us would want to start the season...
