On my little Mercury 15 hp 2 stroke when it is on full tilt up I noticed gearbox oil running down the leg. Upon further inspection it is running out of two holes which obviously should be there (one each side) which you can see in the attached pics.
I am somewhat confused because if gear oil can come out water can surely get in.
Someone must know what these are and if they should be plugged ?
As stated, most likely two stroke residue draining through two drain holes in the leg. Pretty normal. You'll notice it if you run the engine up in a bucket - the water will have a film of it all over the top.