you did not state anything about the engine.
if it's a 2 stroke, it could be the carb or carbs.
a lot of people overlook unhooking the fuel line when you put the boat on the trailer.
i see this all the time and tell everyone , if it's a 2 stroke, it has to be done to keep the gas and oil from seperating while the engine sits.
hers what i have done for many years and it works.
once you have the boat on the trailer and locked down with the engine running, unhook the fuel line and let the engine run out of gas till it dies.
do not hook the fuel back up till you get the boat ready to launch, then prime the fuel bulb and choke the engine if neccesary when you get the boat launched.
mine will always when i launch it.
the problem starts with the new gas we have now.
the alcohol evarorates when it sits in the bowls of the carb and leaves the oil to gum the jets and small holes
in the carb.
hope this helps,
cherryville, north carolina