The line of the spell in question says:
You destroy up to 10 gallons of water in an open container within range.
Range is 30 ft.
I am on a raft. 20 ft below me (and to the north) is a shark. I target the water just below the shark, destroying 10 gallons of water in an open container (the sea).
- The spell destroys the water, instantly as per duration, and the nearby water rushes in, doing nothing more than possibly confusing the shark.
- The water is instantly destroyed, creating a vacuum. As the water rushes in, the change in pressure does say 2d8 (like Thunderwave) to all within say 5 ft of my target location.
- The water is destroyed, turning into a bubble of oxygen and hydrogen, which rises up and buffets the shark. He considers it difficult terrain for 1 round.