There are ways to test the plastic to make SURE of what it is. Search on "identify plastics" or similar phrases Boedeker Plastics : How to Identify Plastic Materials Using The Burn Test
that's one of many sites to give you a start. If you are doing a burn test, needless to say, you do it to a bit you've scraped off, not the whole hatch
Any good plastics shop should be able to identify it for you, but expect that the better grades (UV and scratch resistant) of material will not be in stock and have to be ordered. Same for tints. And most shops will charge you for the whole 4x8 sheet ona special order, and then keep the "scrap". Ask for it, if that's the case. It belongs to you and can be used for other projects on the boat.