If you let it run for 30 minutes and didn't set off the alarm then

1) I'd check the thermostats. Mightn't be something holding it open. Prolly good idea to replace regardless If u don't know how old they are

2) u r getting enough flow from current impeller to cool the motor, else u would have set off alarm in 30 minutes. Look again for obstruction on pee line.

3) regardless of obstruction or not. Replace impeller and housing if u do not know it's history. Cheap ins and peace of mind.

Go to bassboatcentral.com boats and motor page. There is a sticky at top with pics on replacing on a merc. Or at least used to be.

Wannabe...