When sitting out of the water with motor in position I have black oil comming from behind the prop. After use and pulled out of the water the black oil turns to a light gray color. As long as the motor is in position the leaking contunues. I have been told that I need to replace the O-ring just behind the water pump. Any ideas???
Also how hard is it to replace this part??

Sorry, doesn't sound right to me. The part you describe doesn't exist. There are 2 seals below the water pump base but you will need to pull the lower, drain the oil and then pressure test it.

I would check the lower unit grease...It should be full and not watery...

Do you premix your gas.. Or does it have a auto mixer... If you premix your own gas.. It could just be excess oil..

I premix my gas and oil and last week I removed all fuel. Then replaced seal ring on botton end gear oil fill ports becoused I had noticed gear oil leaking from one. The seal ring was missing. I took the boat and ran it for about 30min. I no longer have the issue.

Thats good to hear!!