We have a variety of them on our boats. Here's my take.
The big oval Valley hatches, that close really securely, are generally watertight. They are also so secure that I have been unable to get one open on my husband's boat if he realized after we were off that he needed something from his rear bulkhead while we were on the water. Colder water tends to make hatches suck down harder even in not-cold weather.
My looser and easier to remove Kayak Sport big oval hatch in the Vela will leak some no matter how much I treat it and set it on just right - just due to its enormous circumference. But the water is never enough to be an issue as long as I used dry bags, and if needed my husband can get the thing off on the water.
The big oval hatch in the Vela is also probably about the only thing that allows me to load the boat with much in the rear, because the boat is also very shallow back there. So I don't have depth.
The round hatches in the Explorer LV have been bone dry for its entire life. Except for the day hatch they are the Kayak Sport ones, softer than the Valley hatch, and are easy to get off in all situations. They also can be closed down fine with a whack of the paddle blade if I take off from the beach and then realize a corner wasn't quite down. The stiffer Valley hatches have to be locked down by someone else coming over to your boat if you miss it.
The above Kayak Sport hatch covers sink if they end up in the water. Tethering is not optional. The Valley hatch covers float, so you have a better shot at retrieving them. But in open water paddling, spare hatch covers are a good idea regardless of the flotation rating of your hatch covers.
Boat depth matters a lot in loading. The Explorer LV has a greatly reduced deck height so I would have to use all smaller dry bags even if the hatches were larger. I would also have to separate the tent poles from the tent regardless of hatch opening size to load it most efficiently. In a boat with a taller deck, a bigger hatch opening would allow me to get larger stuff in there intact.
Replacement cost for the bigger oval hatches is pretty painful compared to the 10 inch rounds.
In sum, I'd suggest that you just adapt to whatever hatches are on the boat you like - smaller drybags, aggressive use of same if it is a little wet - all hatch cover solutions have pros and cons.
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