This happens frequently to my Merc 3.3 -- I use some thin wire to poke up the outlet. This works every time. If not you will prob have to strip it down.
Chances are, if your impellor is OK, that your Thermostat is blocked. I know people do not believe that thay have a Thermostat but some do. I used to have a 1980 Mariner, Based on a Yamaha that that did.
Take the head off and dig the salt out of the waterways. Poking bits of wire through the telltale spigot wont help.
I do mine every winter and have used the same gasket with some locktite each time with no problems.
Before going to the trouble of removing the head, try removing the exaust port plate on the side of the block, you will be able to get at any crap in that area which normally causes this problem.