There is a big fundamental difference between open water swimming and pool swimming and it involves not being able to push off a wall in large bodies of water. If you are lucky enough to have the opportunity to train in a 50 meter pool, then you are probably only able to actually swim “long” course for a few months out the year. Therefore the majority of swim training is done in a short 25 yard/meter pool which, if you account for push offs, means we are really only “swimming” around 20 yards/meters per lap. On top of that, we are resting and breaking our stroke every time we make a turn at the wall.
For these reasons, I believe we get a huge aerobic fitness benefit by utilizing some open water swim sessions. Not only are we now swimming continuously for a given period of time (like we will be when we are doing a triathlon or open water race), but we are also able to improve open water techniques such as sighting or pacing (http://www.finisinc.com/blog//drills/pace-yourself-in-open-water).
A good way to break up the monotony of swimming straight for a long period time is to practice surging. Swim for a short period of time (around 2 minutes or so) at race pace. While you are surging, find an object (tree or buoy) to practice sighting towards. When your race-pace interval is over, swim easily for the same time duration. This exercise allows you to practice the surging that often occurs in an open water swim.
By utilizing open water swim sessions in training, swimming straight for long periods of time will seem less foreign when we get to race day. The frequency of open water swim sessions will depend on the availability you have to a lake, river, or ocean. I aim to get one open water swim session in per week to help me stay race ready.