water in lower unit
 

is it realistic to expect absolutely no water in it. How much water in it will it take to ruin it?

Should have ZERO water!! You need to find the leak and fix ASAP Or risk losing the the unit.

When is the last time you changed the lower unit oil?? If you did it in the fall or this spring and this is the first sign of water then it might be as simple as the plastic washer on the fill screw?? These need to be replaced everytime you service the unit.
If you have done nothing then take your prop off and see if there is any fishing line wrapped around the shaft, this is were the line will get in there and cut the seal.
after that the best way is to take it some place and have a leak down test done to determine were its leaking.

Depending how handy you are you may have them fix it as they will have all the tools.