Looks like just an older-style packing gland
leak, it is in a lovely spot to access. Two wrenches are required, loosen the locking nut(the thin one closest to the hull
fitting)a turn or more clock-wise,while holding the packing nut (the wider "nut") from turning. Then tighten the packing nut ,also clock-wise, a quarter turn at a time until the drip stops dripping. Hold it stationary and bring the lock-nut back up to it snuggly. Now run your shaft slowly and carefully observe the gland, you should get one drip every five to ten seconds while the shaft is rotating, just enough to keep your packing material lubricated and cooled. If you find the packing nut will not tighten any more or doesn't stop the leak,or you have no more threads to back off the locknut,you probably need to re-pack the gland(a job requiring hauling), pick all the old packing out of the inside of that packing nut,(may require a special tool), you may have the traditional waxed jute packing or the newer woven teflon material but both have to be the right thickness to fit inside the nut, cut into rings(usually three or four) that just fit round the shaft. You alternate the cut ends 180 degrees as you push them into the nut. Tighten the nut back onto the hull
fitting snuggly, then back it off once you are back in the water, until you get a fast drip.then retighten it as explained above. Most packing glands will run for years with nothing more than the very occassional tightening, if you find you have to tighten it often then you have alignment problems or wear on some other component, ie: v-drive bearings, shaft cutlass bearing(outside the hull)worn out, or a worn shaft in the area of the packing gland
from over-tightening the gland or abrasive(sand) having been pulled into the shaft fittings.