The title is the whole episode. Between streams, Basket filled a big part of the Iron Island chest ring with sand. The plan: a blue glass lip runs along the bottom, water pours over the edge from above, lands in that gutter and runs around in a circle, and the middle stays dry. The sand is there so the water can be cleared out a section at a time. One side effect: the new platform sits right over the beacon, which is why it stopped working.
Water science
The first test goes well. Basket keeps the inner wall of sand, digs out the outer one, and the water pours down into the gutter without running any further. "Better than I was hoping." Plan B had been a wall of sticky pistons to pull a whole section out at once, and for now it isn't needed.
The next section spills. Placing or breaking a block nearby sets the water off again. After testing it column by column, Basket finds the rule: "the corner of where that is has to be the last thing removed." The wall has to stay two thick, and torches go on the pillars that have to stay so nothing spawns on them.
Sand, and what went missing
All that sand has to go somewhere, so it gets shoveled out and packed back in further along. Basket mentions a loss too: "I had a blue shulker box that had all my valuables in it and I somehow lost that." The emeralds are gone too, and there's one mending book left. Later Basket guesses it got broken with a full inventory and left behind.
A viewer asks if blocking the beacon was on purpose. It wasn't, so Basket swaps a block in the platform for glass: "we'll fix the color later." In between, Basket talks through plans: a slime farm on top of the mob farms, built on all four so it's symmetrical even though only one sits in a slime chunk, AFK fishing farms at the bottom, and maybe a squid farm if squid end up collecting in the gutter.
Blue glass stoppers
Another section breaks. Basket's fix is a blue glass pillar at the corner, full height, so it blends in with the water. That works, and then more corners need one. There isn't enough blue glass, so Basket pulls lapis from the ender chest and crafts more. Some sections now behave. Others pour in anyway. "Oh, now you stupid water!" The sand runs out twice along the way.
A break for the tour
When a viewer asks what this all is, Basket takes a break to show it. Four iron golem farms and four mob farms sit under the ocean. Everything they drop rides the item elevator up and gets sorted into the chest ring. There are still no labels, and one whole run of chests is just for poppies. Why put the storage underwater? "Who else has all of their storage in water?" On the far side, the plan is a drop all the way down to bedrock.
Still fighting
Back at the wall, viewers chip in with ideas: a block in front of the top of the waterfall, start with the sand next to the glass, take the inside part out first. That last one lands. Basket had been doing it in the wrong order. Some columns behave after that, but one keeps pouring in no matter what. By the end the sticky piston idea is back: pull the whole column at once so the water never sees an edge. "I think that that would work." The video ends on the credits, with no gameplay after them.