Most peoples difficulties with swimming in open water, is fear, aclimitisation or sighting. As your swimming in a quarry in January I think we can rule the first two out.
For sighting, I cant see that a 300m/10 min swim will help a great deal either, you need to be swimming a proper course, also getting used to sighting with other swimmers around near race conditions when you can't see the buoys so easily.
Unless you're swimming at 45 degree's to the direction you want to go I wouldnt worry to much at this point, as I say 300 swims wont help you improve in my opinion.
Getting back to the question "How can I train for swimming outside", given you have access to a 25m pool, I think you should use it. Improving your technique & structured intervals, ideally some lessons or coaching will help improve your open water swimming far more than a 10 minute splash about in a quarry,