I do believe I'd repair it the correct way as opposed to the above statement and frying batteries and other potential issues with 16+ volts.
As Dave said it;s the voltage reg. "and or" an internally shorted battery with that type system will do the same.
That engine was equipped with a warning horn in the control box. The temp sender to activate it is located on the upper right corner of the cyl. head cover plate. The older systems do Not have a beep when you first turn the key on. The only time you'll here it is if the engine actually does overheat. (providing the sensor & wiring are still ok)
Mercury Quick Silver Mirage Stainless prop.
No dings, No marks, Like new cond.
Part No. 48-13702A4
14 3/4" x 21P
Fits: Mercruiser Sterndrives
R, MR, Alpha, Alpha Gen II, Bravo I
$200.00 pick-up or plus shipping
Retail is over $350.00
Trailer circuit is am entirely separate circuit from your brakes & turn signals. That being said it your vehicle lights are all operative after you replaced the fuse and it then blew when you plugged in the trailer again the short is in the parking light circuit on the trailer. Check the brown wire from the trailer plug back thu the trailer to and including the light sockets themselves for a grounded or chaffed wire.