Personally I'd like a dive but that's mostly for the ease of building in water instead of playing terrain height games. I'm fine with certain materials sinking into the depths, some of which wouldn't be recoverable even with a dive function (like serpent scales mid-ocean. I can see a viking able to hold their breathe underwater, less able to see one proficient in free-diving as if they spent their life oyster farming or something).

That said there are some ways to deal with it if it's relatively shallow still. Jumping in from a higher position makes you sink under the water which can be used to pick up items in water that's just a bit too deep. Jumping in right next to your boat and tucking yourself under it while still submerged can keep you underwater for some tasks, but you will be only just under the water in that case and the camera is not always cooperative while doing so (though the weird jump-swim animation glitch you sometimes get is just hilarious). There's also the option of using commands, but that isn't always an available option.