This sea cock, near the bottom of the bilge below the speed transducer, was the intake for the old Frost water cooled refrigerator that was replaced with a keel cooled Frigoboat unit. This sea cock always leaked, not enough to be dangerous but enough to be annoying. Since it was no longer needed, I decided to replace it and fill in the hole.
After removing the seacock, I ground out a 6 inch square indent on the hull side of the hole about a 1/4 inch down into the fiberglass receding from the 5/8 inch hole. 
The Difficult to access inside was sanded all around the hole and three layers of 8 oz glass cloth was laid on, covered with epoxy resin and held in place with a wax paper covered board. 
With the board as a backing plate, I filled the old hole with layer upon layer of 5/8 diameter discs of fiberglass cloth soaked in epoxy until it filed the hole completely.
Then I laid on ever increasing sized glass patches, 13 in all until I completely filled in the ground out indentation.
I Used a small portable electric heater to keep the epoxy resin warm as it cured in the cool Lake Superior Spring-time temperatures.
After curing completely, I sanded it with 100 grit to fair out the rough spots and now the patch is water tight and ready for paint.