Why does my 89 Yamaha 90hp outboard motor have water in the lower unit?

My grandpa is having trouble with his boat motor. He says there is water in the lower unit. Is there a way to test to see what the actual problem is?

He is going to just try and replace all of the seals, but he wanted me to find a diagram as well.

It is a '89 Yamaha 90 HP E1614
 
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