Sealing a leaky boat hatch can be troublesome. A leak in the hull requires immediate attention, but a leak in a hatchway might seem minor, something you’ll attend to later. The problem is that on calm days, you won’t be in the kind of seas that make watertight integrity critical. A drop of water may be the first indication of a leaky hatchway where the watertight seal isn’t quite perfect. If the leak is left unattended, it might sink your boat.
Mark the location of where the ends of the foam watertight seal meet on the inner side of the hatchway with a permanent marker. Slide a narrow flat blade, like the spatula used to frost a cake, under the end-seam of the watertight seal that's around the inner side of the hatch cover.
When you reach the point where you first placed the gasket into the channel, cut the gasket off with a jack knife so that it is about 1 inch longer than the space in the channel. Force the extra inch of gasket material into the space available to ensure the gasket that goes around the entire channel.