One thing that you always need to keep in mind is this; you can have a thousand identical days in the pool (i.e pace clock, smooth water etc), but no matter how many times you swim in ow, you will never have the same swim twice.
Out in nature, you can't control what is going to happen. It might be a gorgeous day weather wise and then the wind kicks up, or you can have overcast skies and its as smooth as glass. Cold water one day, warm the next. If your in a bay or ocean, glassy one minute 5ft swells a moment later. All this will affect your times in ow. It won't matter in the pool.
When your in open water, try to find bad conditions to train in and see if you can maintain a center of calm throughout. We have a saying in the ow community; "Plan for the worst hope for the best."