I have a 75 Mariner 2 stroke that keeps fouling the same plug. Any ideas to fix this would be great.

What does the fouling look like? Any Chiltons manual or the like usually has a color pic reference in the back. It could be anything from a wire not connected properly to a bad plug to (hope not) bad piston rings.

First, do a compression test and see if all cylinders are within 5% of being the same. If that cylinder is low, you found the cause, so we will need to find a cure. If they varie much, do a good decrab on the motor and check it again. That could bring them up, if not, you may have a problem.

If compression is good, you are probably dumping too much fuel into that one. This could be as simple as adjusting the carbs, worn jets, etc. You might try indexing that plug, improper indexed plugs can cause fouling.

Don't use a Chilton plug color guide, that's for four stroke motors.  Run a two stroke motor to the proper color in there and you will melt the top right out of the piston.   Brown paper bag color is about as light as you want on most carburated two stroke motors.