It is likely to be as Bill suggests. I have had exactly the same problem which drove me mad for eighteen months.
The casing of the Johnson raw water pump in which the impeller is housed was worn by 3mm plus, resulting in the pump cavitating with air on start up.
Once I opened the face plate on the impeller housing the engine ran fine.
I couldn't work out what was wrong until I got a new pump and measured the inside of the housing with a Vernier and it was plain to see the difference.
I had been in Brazil for a couple of years, in the Amazon and many of the tributaries which is what I believe caused it due to the heavy silt. I also went through lots of impellers on the Onan and the Yanmar during this time.
New pump and it was fine.
Seafever SM 425