Por que no los dos? Airtight spaces are the 'best', they create the most buoyancy and (obviously) don't add weight to your grid, but they're also the least accurate and the hardest to change. Even with lots of symmetrical airtight spaces, it'll be hard to keep your sub neutrally buoyant. With a few, though, you can get close then use ballast to get closer. I make a separate conveyor grid attached to a collector & ejector. Put cargo containers as close as you can to the outermost corners of your grid then you can turn the collectors on to get more ballast, you can move the ice around to balance things and you can eject the ice when you no longer need the ballast.

As far as ice vs water density goes, that's not the issue in SE. The ice has weight, and cargos aren't buoyant to begin with.

What I'd *really* like to see is a 1x1x1 small grid 'door' to use to open/close such spaces. Doors are too big and often too heavy. There's one in AWG's conventient weapon pack, but that's ~950MB of tank parts for just one block.

Yes, either a secondary conveyor system or one with rigid sorter protocols is necessary.... Especially if you're making a mining or salvage submersible!

I am on my phone, so can't check, but are the maintenance hatches airtight? (Or maybe those are from a mod - maintenance hatches that can open/close and are fairly low profile)

So if cargo containers aren't buoyant, then Ice is better for weight, yes? Because water is just neutral - it eliminates the positive buoyancy of the airtight space... Obviously balancing and control are issues as well.