I've got a 2006 Yamaha F150 with 1070 hours
I took it in for a service a couple of weeks ago and it turns out there is major corrosion in dry exhaust areas of the powerhead and mid section/exhaust guide. About half the thickness of the walls around the exhaust have corroded away in places!
It seems that some of the early F150, F225, F250's suffer from problems with corrosion in the dry exhaust areas due to problems with the coating/or lack thereof. Quite a few people on other forums are reporting problems and there have even been a couple of law suits in the US about it. (Google F150 Exhaust Corrosion
Might get a few hundred more hours out of it. £3000-4000 to repair, but it's an 8 year old engine. Or buy a new engine....Suzuki DF150TG looks nice
If anyone has an older large Yam it's probably worth inspecting the exhaust when the legs next off!