Need some help with a water pump issue with 20hp Mercury

I recently replaced the impeller and supply tube seal at the bottom of the supply tube on my 1986 Mercury 20 hp outboard. Since mounting the foot back onto the lower unit I am not getting any water to pump. I dropped the foot again and noticed the supply tube is not holding on the top end. Is there another rubber bushing/seal at the top end and if so, how complicated is this to change? Previously the tube would stay in the lower unit and disconnect at the rubber seal on the top of the water pump housing. I'm assuming because it is not sealing at the top end I'm unable to draw the water to the top end and push it through the block as it should.

Powerhead has to be pulled to replace upper rubber grommet.

I can only guess based on your description, but I suspect you are calling the drive shaft housing the lower unit, and you are calling the lower unit the foot. Either way, we need to try to use the same terminology so that any help or instructions you receive, are not made more confusing. If the upper sleeve/tube pulls off with the water line, there may be a way to replace it, if it's like the Merc's I've had and work on. You can slip the tube onto the end of your water line just enough that it will stay lined up properly. Now wrap the top of the water tube right next to the sleeve with a few wraps of tape. A small build up of tape prevents the sleeve from sliding down onto the tube any further and will allow you to use the tube like an extension handle to push the sleeve onto the upper connection. Once you have pushed it all the way onto the upper connection, pull the water tube out and remove the tape. Then you reinstall the tube into the upper sleeve and then install the lower unit.

Any time I remove my lower unit, the water tube usually stays with the rest of the motor (inside the shaft housing). But on the occassions that it pulls out with the lower unit, I just reinstall the line back into the upper sleeve and go on with what I was doing. I've only replaced the sleeves when they have split or otherwise become unusable.

Had the same problem with mine. After hours of trying to figure out what i did wrong and blowing air through the outlet hose i finally took the lower unit off and put the metal tube in first and then gently put the lower back on making sure the tube went into the grommet coming out of the water pump. Tightened it all back up and shot a nice stream of water. i checked the schematic for that motor and i couldnt see any bushing for the top of the tube to go into. did this at the beginning of the summer and i havent had a problem with it since.