If you know the depth of the water, then lower your line all the way to the bottom and count how many turns of the handle it takes to get the hook from the bottom back to the surface. Then you can estimate how many turns to get to the desired depth. For example, if the bottom is 30 feet deep and it takes 12 turns to bring the hook to the surface, then half the depth (15 feet) would be reached by reeling the line in 6 turns. I knew that college algebra would come in handy one day!
Another method I use with an open-face reel is to:
1. pull out 25' by hand (estimating one foot per pull), plus one or two feet
2. clip the line to the line saver found on the side of most spools
3. and then crank the handle once to wrap up the extra 1-2 feet you pulled out in step 1 (if you don't do this step you stand a chance to have the clip break off or to have the line cut by the clip when you set the hook on a good fish).