system
Given the following post, respond as if you're commenting on Reddit:
human
[Pirates of the Caribbean] How were Jack and Will able to use a capsized boat underwater?

Wouldn't the pressure that would push it to the surface be greater than their combined body weight?
gpt
It's all about that neutral buoyancy.

Put the boat in the water.

Roll the boat so that it is full of water.

Fill the boat with rocks or lead shot until it sinks. Secure the lead shot in containers in the fore and aft.

Roll the boat underwater.

Fill an airtight bladder with air. Mostly airtight is acceptable. (Denim would work, so canvas sacks would as well.)

Push the bladder under the capsized boat.

Release the air.

Repeat.

One person needs to keep the boat from rolling and releasing the air back to the surface.

Once the boat reaches neutral buoyancy, it would actually be relatively easy to walk it across the cove floor. Visibility might be a problem. Depth is definitely a problem, a depth of more than 35 feet might be enough to cause Decompression Sickness.

The more significant problem is the lack of oxygen. For every cubic foot of air you put into the boat, you are displacing 45 pounds of water. To balance this you need to add eight pounds to the (now upside down) boat's weight, or put 45 pounds in your pockets and hold on really tight.

Now a single cubic foot of oxygen will last about 10-20 minutes (exercising, talking and anxiety all increase oxygen intake). So, assuming they start with fresh air, they don't need to travel more than 30 minutes across the bottom of the cove, then they need to load the boat with about 6 cubic feet of air. (Which is about what we see inside the boat actually.) And this would be the weight of 10-20 cannonballs. At least those are readily available.

However, there is a bigger problem! CO^2 poisoning. Unless you start with pure oxygen, (not recommended at higher pressures!) you'll actually start to suffer from CO^2 poisoning in 1 /5th of the time it takes to actually breathe all of the available oxygen. However, there's an explanation for that too. See, in high pressure situations like the bottom of the ocean, the CO^2 can be reabsorbed by the saltwater, allowing one to live almost indefinitely in the cold and dark.

What isn't explained is why no one noticed all of this preparation, or the giant wooden crab invading the cove. Seriously, that water is like glass.