Open water swim workouts

I live in Northern Virginia and rarely get an opportunity to OWS outside a race.  Today, I'll be swimming in a flooded quarry that is set up with a 600 yard loop.  Last time I was there, I just swam seven loops nonstop (except for a two-minute break to add more Body Glide around my neck).  I mainly did that for the confidence boost to confirm that I've got what I'll need to do IMLP next summer.

Anyway, when you OWS, do you go nonstop or try to do workouts to mimic what you do in the pool?  Today, I think I'll swim a littler faster and take a 1 minute break after each loop for 6-7x600. 

On a related note, next summer I'll be on vacation from July 3-10, just a few weeks before IMLP on July 25.  I'll be on a lake in northern Minnesota.  OWS will be wonderful--it's 1250 meters across the crystal clear lake each way.  But should I be thinking more in terms of intervals based on time rather than continuous swims?  (I know, why am I thinking 10 months ahead ?)