Draining the water in Sonic Frontiers is one of the first puzzles you encounter when you arrive in the second zone, Ares Island. It's a bit unclear what you actually need to do to clear the pool that's blocking your path. There's a cannon, a floating diamond, and a bomb—all the makings of a good puzzle.
If you're struggling to figure out how Sonic Frontiers fast travel (opens in new tab) works, then this should help you out. Otherwise, here's how to drain the water on Ares Island so you can keep exploring the zone.
How to drain the water on Ares Island
First off, you need to have claimed a Chaos Emerald from Ares Island and completed Knuckles' second set of memories—just like the Grave Mystery puzzle (opens in new tab) on Kronos, this one is progress locked too. Once you've done both a bomb will appear on the opposite side of the pool to the cannon and you'll be free to attempt the puzzle.
To drain the water on Ares Island, you're going to have to use the big cannon that's next to the pool, though you'll need something to load it with first. This is where the bomb comes in. You can use your cyloop skill to draw a circle around the bomb to pick it up, and once you do, you'll have 30 seconds to get it back to the cannon and shoot.
Once the bomb is in your hands, run back over to the cannon and load it in. Now you need to aim it at the floating black diamond attached to the chain—specifically the red eye-like symbol at very centre. Shoot this and it'll cause the pyramid to rise, pulling the plug and draining the water from the pool. Now, you can drop down into the space below and keep on exploring.