what do you need to know?
Alpha drives are for Mercruisers. The gearcases were changed to different designs over the years. Hopefully you have the exact manual for that gearcase.
Sounds like you have a water leak. If you haven't taken it apart yet, I think it worth a try to replace the driveshaft seals and the vent and fill plug seals and retest it. I have found most leaks were from those areas. Obviously you need to clean up the driveshaft so you do not tear the new seal.
If you are just rebuilding the waterpump, there is no need to shim the pump.
Just be aware that Merc changed the orientation of the driveshaft bearings, in different year gearcases, and therefore not all gearcases are shimmed the same.
When you say rebuilding, you mean using the same parts, correct? If I replace the watepump parts would I then need to resim and measure the gap?
if you can see bearing on input shaft u have removed the base plate in which case u will need to check shims... if u have not removed the lower plate than ignore the shim part of things
You can replace any part of the water pump, except the base, without worrying about having to adjust the shims. The water pump consists of three main parts, the upper housing which contains the impeller and S/S cup insert, the impeller itself, and the base which contains the seals which seal the lower unit.
In a typical "rebuild" you would replace the upper housing with S/S cup insert, impeller, S/S wear plate and all the gaskets. Only if you are getting water into the lower unit oil would you need to remove the water pump base and check the seals.
If you remove the water pump base to replace the seals and then reinstall the SAME base, then you do not need to check shimming. Only if you REPLACE the base will you need to verify and/or adjust the shimming.
