That's pretty slick. You've got me thinking about getting out the sawzall.
I hadn't thought about using a big watertight hatch for that. How is it as far as walking on it? Did you do anything to reinforce the deck around the opening? Where did you get the hatch?
I had worked out in my head a way to cut out a chunk of the deck and then frame in around the edges so I could use the cut out for the hatch cover. Your idea seems easier.