Have you tried emptying your lungs? It sounds crazy, but you actually don’t have to completely fill your lungs with air, just enough so that you can last a good while underwater and so that you’re no longer buoyant. It takes some practice, and you might panic a little bit at first, but once you get used to that, it just becomes second nature.
The only times I hold as much air as possible are when I’m trying apneas (holding my breath as long as possible), but I stay near the surface for that and I remain inmobile. (Also, make sure there’s more people with you and that you’re signing that you’re fine)
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