I wouldn't suggest treading water. You use just as much energy treading as you would swimming. Switch up your strokes. Breast stroke is a great open water stroke because you can see what's going on ahead of you rather then poking your head up every so often as with the crawl. If you get freaked out about not being able to touch and panic, get on your back and keep moving. Don't panic. People don't drown because they can't swim, they drown because they panic. Well, if you can't swim there is a good chance you will drown but in that case we wouldn't figure you'd be hanging out here. Anywhoo...Most open water swims have people in canoes you can grab on to if you need to rest. The can't advance you, but you can hang for a bit. Practice in the lake/ocean the event will be held in, in different conditions if you can. Just relax and you'll do great.