Yes, we are trying to displace water with air to provide a positive buoyancy and two one gallon milk cartons will provide enough positive buoyancy to keep a two hundred pound person neutral to slightly positive in salt or fresh water. No...it will not float them above the surface. We are trying to provide enough flotation to keep the boat from sinking, it doesn't take all that much to accomplish that.
In an emergency a pair of pants with the legs tied in a knot at the ankle and simply pulled tight at the waist with air inside will support the average person and keep them from sinking. This is one of the life saving techniques taught in many areas. The amount of air captured in the pants isn't much more than provided in a pair of milk cartons and fortunately the air stays in the milk cartons better than in a pair of pants upended.
These boats will sink because of the added weight of the glass, paint and the extras that are added to them. But the boat itself is relatively positive in its ability to float, even when filled with water. Add something such as a closed cell foam or even several sealed bags of air and the boat will stay afloat even when filled to the gunnels with water. Enough to support a person, but not keep them high and dry. Just remember that any foam has to be closed cell, not open cell. I have seen lots of people use upholstery foam and think it will provide flotation, but it is open cell foam and it soaks it up like a sponge. Same with most of the foams used in packaging. The best flotation comes from a closed, sealed area without anything in it. (A flotation chamber) Ice chests when empty are wonderful, and in an emergency, empty the contents and close it up, it adds a lot of buoyancy. Even closed zip-loc bags filled with air help.
I saw one small 16' glass sailboat boat floating at its rails and when pulled out and the water dumped out of the boat the chambers under the edges of the deck had been filled with foam peanuts from packaging, and they were packed with them in plastic bags. The total amount of area was roughly equavalent to 4-5 milk cartons worth of area. It provided enough additional buoyancy to keep the small cabin top and hand holds above water and the three persons alive until they were picked up.