I have a Mariner 8hp which overheats after about 10 minutes. The tell-tale
stream turns to steam and the engine stops shortly after.

I started at he bottom and checked impeller and pipe up to powerhead all
working fine - good stream at idle and for about 5 minutes at high rpm so I
think the water flow is ample but engine gets hot and it can't cope.

I have taken off head and checked passages, ditched thermostat and removed
grit from water jacket around cylinders.
Some aluminium has chipped off around the sleeves exposing the actual
sleeves to the water. Could this be what heats the water up to such a high
temperature?