The older way of doing this is simply to have several pillars in line, each one a bit higher up than the previous one as you move away from the shore. I think they're usually painted white with a red band or something.
From a ship, if you see the pillars in a straight line ahead of you, you're in the channel; if the rear pillar is to the left of the front one from your viewpoint you're too far left.
The vertical version of this is the visual approach indicator for aircraft: red and white pairs of lights with flaps at different angles so that if you're too high, you see all white, and if you're too low all red. On the exact glide path you see both red and white.
From a ship, if you see the pillars in a straight line ahead of you, you're in the channel; if the rear pillar is to the left of the front one from your viewpoint you're too far left.
The vertical version of this is the visual approach indicator for aircraft: red and white pairs of lights with flaps at different angles so that if you're too high, you see all white, and if you're too low all red. On the exact glide path you see both red and white.