loud a$$ buzzer going off?

Out at the lake today for the first time this summer and while waiting for a buddy to return from parking the truck, I was idleing. about 100 yards past the 5MPH bouys a loud buzzer went off? temp read about 190-195, threw some oil in it and no help once again within 45 secounds the alarm went off again. Just a guess, could it be the impeller? and how much work is it to change?