A Saltmarsh arcade
A heist on one breath. Creep past the sahuagin, work the oysters and locks, and get back to the skiff before you drown.
Tap to swim, hold on loot to work it, hide in kelp. Swim back up to the skiff to bank. Back to the arcade.
The crabbers' rules
You start under the skiff. Tap anywhere to swim there in a straight line, or keep your finger down and drag to steer; a ring marks where you're headed, and you slide along rocks. The cove is a stack of shelves, each one screen and about 8 fathoms deep. A crevice in each floor drops you to the next. On a keyboard, swim with the arrows or WASD. Esc or P pauses.
The bar top right drains every second, faster the deeper you are, even while you hide or work. The gold tick on it is the breath it takes to swim straight back up to the skiff from here, if no patrol makes you wait. The bar turns gold when you're cutting it close and red when you won't make it. There's no Surface button: swim back up through each shelf's opening and out under the skiff to bank (once you've been at least one shelf down). Run out and you drown with everything.
Sahuagin. From shelf 2 down they patrol, and the green wedge in front of each is what it sees. Rock blocks it. Stay in a wedge and a ? fills over half a second; when it turns to ! the sahuagin hunts you. Getting caught steals 30% of everything you carry and 3 seconds of breath, and throws you back to the shelf's entrance.
Kelp. While you're in a kelp bed you're hidden and nothing can spot you, though a hunter can still bump into you. Break its line of sight for 2 seconds and it gives up.
Noise. An oyster clamping shut, a missed lock and a snagged net all make noise. Sahuagin nearby swim over to look.
Traces. Opened oysters and chests stay open, and a sahuagin that spots one is alerted for the rest of the dive: faster, sharper-eyed, and marked in red. Every chest you crack on the way down makes the climb back harder.
Press and hold on something within reach to work it (hold Space or E on a keyboard). A ring appears above it; let go to release. You can't swim while you work, and your breath keeps draining.
Oysters. From shelf 3. The ring fills: let go in the gold arc for a pearl, or in the thin black arc near the end for a black pearl, worth three. Too early and nothing happens. Hold too long and it clamps shut: 3 seconds of breath, noise, and it's closed for good.
Chests. From shelf 6. A three-tumbler lock: the needle turns while you hold, so let go in the gold window to click a tumbler. Miss and the lock resets, noisily.
Nets. From shelf 3, strung across gaps. Hold for a second and a half to cut one. Swim into one and you're snagged for a moment, and the floats rattle.
Air pockets. From shelf 2, under ledges and often on a patrol's beat. Hold to gulp breath back, up to 8 seconds a pocket.
Coins. 10 gold each. Swim over them; they make no sound.
A pearl is 40 gold, a black pearl 120, a chest 150. Loot is worth more the deeper you find it: double at 25 fathoms, triple at 50.
Swim out under the skiff to bank everything you carry, plus 10 gold for every fathom of your deepest point. Drown and you bank nothing.
Quiet streak. Every piece you take while no sahuagin on the shelf is hunting or showing a ? builds your streak: every 2 pieces add ×1 to what loot is worth, up to ×4. Being spotted or caught resets it.
Holding your breath.
The cove keeps it.