Re: 25HP Mercury Change Impeller
That impeller has seen better days, but it still should have pumped a small amount of water.
Since you have the lower unit off, this would be a good opportunity to blow air up the water tube and/or hook up a hose to the tube and flow some water thru the motor. You'll get a good idea if it's plugged up or not.
The thermostat, if there is one on your motor, is located on the water jacket cover at the rear of the block (it's not a cyl head on this model) and is under a smaller cover, with 2 screws holding it on.
Can't tell without a Serial # whether you'd have one or not, looks like some do and some don't.
At any rate, if there isn't a smaller cover back there near where the telltale hose connects to the block cover, you don't have one.
If you do, pull the thermostat cover for inspection. Flush water thru the copper water supply tube with the cover off and you should get a strong flow thru the open hole.
If the thermostat looks nasty, replace it. If it's clean, might be OK. You can test in a pot of hot water, suspend the 'stat along with a candy thermometer so that neither touches the sides or bottom of the pot.
The 'stat should be fully open somewhere between 120-130 deg. If it's passing 140 deg and the 'stat isn't even showing signs of opening, it's shot.
When you get the new impeller installed, fire off the engine in a bucket (or with flusher if you have that) and see if it'll pump water out the thermostat hole. A good check of water pumping action. Once you reinstall the 'stat let it idle while monitoring the temperature of the exhaust manifold (the big flat cover on the Port side of the motor). If it gets hot real fast, shut the motor down. It should be fairly cool and only get warm-ish gradually if at all.
Anyway, between those 2 areas (impeller and thermostat) you should find your problem.
One word of caution if you have a 'stat is to watch out for the cover screws. If they're tight and won't break loose with normal hand tool force, heat 'em up with a propane or MAPP torch to help bust them loose. You don't want to be drilling/tapping/helicoiling busted screws in an aluminum block. No fun at all !
HTH & let us know how it turns out...........ed