Ignore the floating gems and red skull for now and swim across the pool to the stairs. Go quickly since the pool is infested with crocodiles.
You can then shoot the crocodiles from the bank.
Move around to the near right corner to find a metal ball. Roll it to the left onto the square pressure pad.
This raises a series of square columns from the water, as well as a wooden ledge beneath the red skull. Now you can run across and collect the gems.
Go up the stairs a bit. Then turn around and jump to grab the wooden ledge. Pull up and take the red skull (3/5).
The co-op version is the same. You can use the metal ball or just have one character stand on the pad while another retrieves the skull.
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