I used to swim laps everyday and noticed how much more energy it took to swim the same distance in the ocean, much of the fatigue attributable to not being used to the unpredictable environment. But I did notice that once I relaxed, and found my stroke and breathing rhythm, swimming (parallel to the shore) was much easier, even in a slight chop. Having said that, when you do any activity long enough you can feel when your body has reached its half-way fatigue point. I'd say, along with all the other great safety advice, make sure you don't go past your half-way point, especially in the water.