I recently bought a boat last night and took it out today. I can cruise happy at 14kts but if I go over that for an extended period of time an alarm comes on until I go into neutral and idle for approx 1 min.
my guess is it's getting too hot when you have it wide open for a log time - does it have a good water stream tell tale ? Sometimes over rev's will sound the alarm also - and the oil tank has an alarm too