Plan the dive from the tank backwards. The empty propane tank is counted as is, since most players find their first one in a barrel. Unfold it and the bill turns into plain metal, 175 frags and a little scrap, with the tank's own craft at a Level 2 workbench.
The catch is who else wants that tank. A coastal base usually spends its first one on a water purifier, and flame turrets take two each. When tanks are short, the purifier wins: you drink every day and hunt sharks only on some.
Research is cheap on both sides, so a group that dives often learns the tank too and stops relying on barrels. Craft a few speargun spears before you get wet. Above water this gun does not fire at all.
