My son started playing at age 6. He does it on his own. He showed me how it works.
I’ve never been trapped in an underground water pool… but, anyway, if you get lost underground, you can dig a “staircase” up. Ground/sea level in most of the games I’ve played are around height-level 62-ish. Hit F3 to see your position coordinates. The Y value is your head height. (Each player is two blocks tall.) You can see these coordinate numbers change as you move (or jump) about.
Mine the block right above your head (so you can jump), mine the block right in front of your face, mine the block above that, hop up (forward) one block, repeat. You’ll be mining three blocks for every block you move forward and up. Be careful, though, in case you come up into lava from below, you must be ready to block it off (with the blocks you have been mining) before it flows down on to you. If you hit a lava pool from below, you will have to mine around it, exploring for it’s edges.