Excellent point, however, with bottles and a cauldron, you can get a second bucket of water from your first, therefore infinite water can come from just one bucket. That doesn't matter too much, however, as infinity divided by two is still infinity (E.g. every even number).
Lava, even red hot molten lava like in Minecraft, is much denser than water (like 3100x if a remember correctly) and in the nether it spreads out just as much as water does in the overworld, we can also see that a lava bucket from the nether is the same as one from the overworld. So the player can actually carry much more than you have calculated. But one lava bucket can be cooled into one obsidian, which can be compressed into a ender chest, requiring 8 obsidian, the maximum obsidian of any crafting recipe (I always use to ignore the ender chest when thinking about how much weight the player could carry because it's more like a pocket dimension then anything, and I never considered the weight of the materials till now). Ender chests are also stackable so you can multiple that weight by 64, you could also put them in shulkers (which are not pocket dimensions) and in the new update shulkers can go into bundles, in a way, making them stackable too. This could greatly increase your maximum weight calculation, however there is more, the player's arm makes the same motion mining, punching, and rowing. Secondly a boat can have a chest, filling this with bundles full of shulkers full of ender chest would not add to the maximum the player can lift but it does add to the max they can move. Finally F=ma, if a player is in a boat on blue ice there is minimal friction so you can multiple the weight of all they are moving in the boat by the acceleration of the boat giving you the force their two arms are producing. Dividing by 2 you get the force they player uses every time they punch or mine and because a player's punch does 1/2 a heart of damage multiplying by 20 gives the force required to kill the player (note the force would be many times a NUCLEAR BOMB focused down to the size of the player's fist). PLEASE!!! calculate the exact force and mass. Also hypothetically, if the lava is compressed as much as it is, yet it still bright hot could you calculate the temperature it would need to be at to remain liquid and hence the black-body radiation it would produce. if you could, and it was much higher than the temperatures produced by a nuke and much more radioactive you would have just proved the player can survive a nuclear bomb without a scratch. PLEASE, I SUBBED JUST INCASE YOU MAKE A SECOND VIDEO ABOUT THIS!!!
I'm not sure if you took this into consideration or not but the water wouldn't be able to spread at the very bottom layer of the world because it would need a block under it to spread and you can't place a block in the void
