The fluid is no problem, but you need to rig up a oil pan or something that can stay there a while. Better yet run the boat to heat up the fluid, put it on the lift then drain. You need to remove the prop first then there is a drain plug at the bottom behind the prop. Take it out then crack open the vent plug (Same looking thing) on the right side near the top. Make sure you have a oil pan that will take at least 4 quarts. If cold I let it drain overnite. I keep the drive down and level. To put the fluid back in get a pump from any merc dealer and follow the directions, It is like a soap pump bottle with clear hose. you screw it on to the top of the bottle an pump away. Usually 3 - 4 quarts. I leave the vent plug off until the fluid starts appearing at the hole then plug. Keep pumping about another quart and watch the resivour tank, Stop pumping about 1" below the line, It seems to keep expanding for a minute or two.
The impeller is on the bottom left of the engine (usually) and it is a belt driven pump. It would be easier for you to remove the whole pump then replace the impeller on a bench, Or if you want take it to a merc dealer and let them do it. It is pretty easy though. Removing it is the hard part. I am assuming you have a Merc big block,, right??