If you are refering to your lower unit as your "foot" then it sounds as if you are getting water in somewhere. What type / size of motor are we working on?
Is this inside the exhaust? if so, nothing to worry about alrhough it is normally black sludge. Lots of idle time or did you just run on the hose? If it is inside the gearcase where the gearlube is then you definitely have a problem that needs attention. HInt: The more specific your information is the better the answer we can give ya'
This is a '74 7.5 Mercury and its not in the foot its in the shaft where the exhaust should be most of the time its at 1/2 throttle or less running in a small creek.