If you look at the marine catalogs of yesteryear you will see that "portlights" are fixed glass prisms or transparent material that is permanently mounted mostly in a deck to allow "light" into a dark area of the boat. They do not open. A "port hole" is an "open-able" window in the side of the vessel. In the ancient days they were round but now are mostly rectangular.
Yep, that's how I remembered it... didn't realize it hand changed...
CHAPMAN PILOTING, 58th edition, page 20: Portholes and Portlights
On a boat, a PORT (or PORTHOLE) is an opening in the hull to admit light and air. The glass used in it to keep the hull weathertight is termed a PORTLIGHT if it can be opened, and a DEADLIGHT if not.
That's clearly an obsoleted reference. These days, the correct term would be "the glazing used" since glass is not the only glazing used. And under Chapman's definition, then hatches must be "ports" since they are in the hull for those same purposes. Or if the deck isn't part of the hull, what of the windows in the cabin sides?!