The slipping part - you may want to have checked...if you feel it's slipping...that part I did not focus in on - I am with J.M on the fluid change. If you have the 450 transmission - it has a filter... hard to get to - have to pull shaft to reach - even if you have small hands. A prop strike is not good - how hard did strike the ground...did prop get damaged? Is shaft still straight?
When you say slip - while underway - loss of speed and increase in rpm's?
...A bad day water skiing still beats a good day at work...1995 Pro Star 205....