Put boat in water yesterday for first time this year and was running
fine for about a quarter of a mile and suddenly the prop seamed to
cavitate. No forward progress (only going about 5 mph). Got boat back
on trailer. The boat and outdrive are 6 years old. Outdrive had
impeller replaced last spring , no problems until yesterday (impeller
didn't really NEED to be replaced).

What is the most probable cause of failure (only have neutral)???
Thank you....