"D.B." wrote
I have been getting about a half a cup of water
in the foot after about every 4 hours of operation.
I have gear oil from the foot in the bilge.

The easiest way to locate a leak is with a pressure test.
http://www.mercstuff.com/pressuregage.htm Apply a little air pressure and
watch/listen for where the oil and water squirt out.

without having specialized tools, is it practical
for me to replace the seals myself

If you replace the entire drive with them, yes. That is to say, you can take
the drive off with common tools but if you want to rebuild it, you'll need a
boat load of special and expensive mercruiser tools. I'm pretty sure if I
was in your situation I'd weigh the cost of all the tools and parts and
aggravation against one of these:
http://www.sterndrive.cc/alpha-one-s...e/alphaone.htm