Sea cock
You could remove it temporarily so that you can either work on it, or replace it. If the boat is in the water, just put a cork or anything that you can use for a stopper, in the through hull fitting. You may try soaking it in fresh water or try to take it apart to free it. If it's a metal fitting, try to take it apart and remove any rubber washers in it. Then maybe you could try to boil it in fresh water, or let it soak in Liquid Wrench, WD-40, or Kerosene for a while. Once you get this thing freed up and in working order, it's always a good idea to keep a cork on a string tied near this sea cock in the event that something ever goes wrong, and you need to plug it in a hurry. With fittings such as these and even the metal adjusters and hold-downs on your hatch, a little WD-40 and sometimes some heat from a torch along with some wrenches can free them up. You just have to take the device off the boat and remove any plastic, rubber, and etc before you do it. I've been able to free up the metal parts on my forward hatch in this manner with Liquid Wrench or WD-40 along with heat. The plastic knobs can be carefully separated from the metal inserts on the adjuster bolts. O'Day hatches are notorious for seizing up. If worst comes to worst, you can always replace it with another sea cock and still be back in business.Joe