yamaha f25 warning light

[corgi]

Hi just bought a shetland 570 with a 2003 yamaha F25 4 stroke engine will tick over all day happily but 2 minutes after getting under way the engine warning light and alarm activate and the revs reduce, the online manual says this is for overheating or low oil or oil pressure but the water tell tale pumps very well and does not come out hot, also the oil is new and full and when returned to tickover all is well any advise welcome.

 

[PaulJS]

Could be a stuck thermostat or blocked water passages.
I'm not familiar with these engines but I'd guess that someting's restricting water flow around the head and cylinder passages.
You can get chemical solutions for flushing out the passages, they've been mentioned on here in the past, but I can't remember which one was deemed the best.
Good luck.