I took the strap off an old life jacket, cut it, and attached either end to the bottom of the passenger seat on either end. The other ends met like a seat belt across the passenger seat. Put the child car seat on the boat seat and buckle in as normal. (when not it use just leave under the seat) Baby gets to watch some skiing... Bruce is right. They usually pass out pretty quick.
Do use a shock tube when doing this. We are not a family of handle poppers, but better safe.