Andrew, I replaced both of my aft cabin ports by removing the screws and then using a flexible bladed putty knife. By flexing the blade and inserting it inside the hull between the window and the sealant and working it around the window I found that they came out fairly easily. In your case if you are in the water you will need to drill a few holes in the old window to keep it from being an underwater port for fish. What I found was that the factory used a strip of sealant that was attached the hull by double sided tape in addition the sealant. What I did was to hold the new window in the port and trace around the opening from the inside. Then I removed the new window and cut through the protective paper on the port and removed that paper from the inside of the port. This will give you the area that must be sanded to allow the sealant, Dow 795 black, to adhere the new window to the hull. At the same time by using the old window as a template you can drill the holes for the mounting screws. Apply the 795 to the hull where the port will be placed, suggest that you also use masking tape around the window on the outside, and then gently place the new window opening and press to distribute the sealant and install the retaining screws. I also applied a bead of 759 around the edge to the window to insure a good seal. After that, peal the protective paper of the window and hull, stand back, and admire your great work.
Bob