I see no evidence of any kind of 'through hull fitting'.. looks like a glassed in stub (that may originally have been a f/g tube to the cockpit sole if it's a cockpit drain) Looks like someone somehow fit that gate valve into the f/g stub.
That's the wrong kind of valve, by the way.. a 90 degree ball valve is a better style for that job. Cutting out that stub and installing a real thruhull valve would be the 'proper' fix.
Have a look here:
Replacing Thru-Hulls and Seacocks Photo Gallery by Compass Marine How To at pbase.com