My father has a Selva 350 with an oldish Mariner 30 on the back (I believe it is a re-badged Yamaha engine). It is a good combination that has been a lotof fun over the years.
The boat /engine hasn't been used too much recently (it needs tides, weather and the right people to all be right and I had some heart surgery which meant no boating). Discovering problems and then troubleshooting them is somewhat a drawn-out process.
A while ago the engine shredded an impeller. Only the hub and a couple of blades were retrievable from the lower unit - the rest had vanished. A new impeller didn't pump well so the engine went off for repair. The powerhead, middle-section and lower leg were separated and the powerhead stripped down and rebuilt. Because of one thing and another it didn't get used at all last year (I know, I know...
The engine starts well and pumps from the tell-tale. However, after being loaded for a minute or so the buzzer sounds, the warning light comes on and the revs drop (which I believe are all to do with an overheat being detected). The oil level (that can also trigger the warnings) is fine and the level sensor tests OK.
My theory is that somewhere there is a bit of impeller stuck in a cooling channel where it wasn't visible during the rebuild. Under load the water flow isn't sufficient to cool the engine properly. Is this a valid theory and if so, what (if anything) could be done about it? Suffice to say the work is out of warranty and the boat / engine are in a very awkward location (on a beach with no trailer access, etc.,).