Goop up the area including the nut and washer with 3M 4200 and call it good.
...put the plug in the transom and put your garden hose in the boat. Soon you will identify the leak.
...If it’s a slow leak indeed I’d try 4200 as noted above...
Osky
Do NOT follow any of the above advice.
If a bolt that is supposed to keep the water out is so loose that you can wiggle it, and you really don't understand what to do about it, please seek professional help. If you refuse to seek professional help, at least post photos so someone here can advise appropriately.
