Trawling must be out in the Open Sea. You cannot do it local. Look for the Boat Icon Below to go to sea! When you are in 3rd person view you will see an icon just as you can "Dock" but instead will take you into the open sea.
To begin trawling you MUST have 4 Crew Members!
The Buttons on the bottom from Left to Right, Controls Left Pulley and Center Button is Drop/Reel Trawler and Right Side Pulley
This is the Trawler Screen, You can press E and get a bigger Picture
You can note by looking at the screen \ / DP (M)
This is the depth of the Left line as was as the Right line located next to it.