A [[dredge]] is a tool used to dig dirt from under the water level. Maximum depth may be determined by your digging skill. Dredge quality is also known to affect maximum depth. You cannot dredge at a depth greater than 180 dirt below sea level.
You need a boat to dredge; any boat will do, there is no specific "Dredger" boat type. When not in a boat, you can only dredge to the same depth as you could dig with a shovel, which is 8 7 dirt deep below water level.
To dredge, drive a boat to +-corner you wish to dredge and disembark, drag the boat to corner you want to dredge and stand still for a few seconds, embark the boat, activate the dredge, right-click the boat and select "Dredge". Boarding the boat as a passenger may help you stay on top of the tile easier, without drifting. Dredging makes the tile you dredged steeper, just like normal digging.