Tough to explain in a sentence, so I will use bullets.

- Submerged (diving) under water for a long-ish time.
- Loot Spawn Point appears.
- Swim over (while underwater) and press flipper button to activate loot.
- Character hops out of water, still moving, as the original "Dive" has run out.
- Yellow exclamation point appears, and loot may be gotten (or not, depending on manual dexterity of player).

Specifically the problem that I noticed was that you can run out of 'breath' and pop out of the water before ending your loot dive.

Oh, and I've never heard it called a hay barrel. Learned somethin new today