You are right to query whether the the engine
will tolerate a 20 degree C increase in cooling
water temp in light of intermittant overheating
in the cooler BC waters you have already experienced.
You are almost bound to have issues with overheating with this level of increase so need to sort out the the reason why now.
You should start with the easy checks and eliminate problems or not to spec issues as you find them one at a time.
First check the engine cooling
requirement from the manual. Lets say 20 litres/min at 2000 rpm
. Then check the actual flow through the engine by measuring with a bucket and stop watch at the exhaust
outlet with the boat at the dock/berth, in forward drive and with the engine at its saturated running temperature and stable.
With a known sea water temp, a known running temp and a known engine throughput in litres /min this will tell you whether the engine will handle the 20 deg C increase you expect to see. Your engine temp should be less than 80 deg C at normal running temperature.
While the engine thermostat should handle the temp change, if you already see intermittant overheating it suggests that the cooling water throughput is probably less than it should be now.