Open Water Swim Training

Go with a buddy and swim parallel to shore. You can swim just fine in 3 or 4 feet of water, it's the same thing.

Here is what I did.  Grab a couple friends and go out to the middle of the lake on a boat. Jump off the boat and swim (or sink). Do that a couple times and you'll be golden.

Actually, swimming from dock to dock with a friend in a kayak really helped as well. Also, learn to roll over on your back if you get in trouble. The more exposure you get to open water, the better you'll feel - pretty soon it'll start feeling like the pool. Good luck!

I just posted a very similar thread within the past couple weeks. Lots of responses.
http://www.beginnertriathlete.com/discussion/forums/thread-view.asp?tid=166109&posts=29&start=1

In the end I ended up hauling my wife along to at least be able to call 911 and point to the spot where I went down.

If you must swim alone, make sure to know the spot well - what is the water temp, currents, marine life, submerged vegetation, etc.  I always wear a bright swim cap and swim parallel to the shore if alone.  Depending on the temps, a wetsuit is a good idea as the extra floation allows you to easily float on your back.

I'm not sure what a rescus can is, but have you looked at the swimsafe- http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=980?

A rescue can is a bright red plastic float (as seen on Baywatch) that lifeguards use. I think that would be equally as useful to alert boaters that a swimmer is in the water as it is brighter and bigger than you little head. You could just leash it around your waist with about 6 or 8 feet of line. Of course, that line could become a hazard itself if it gets hung on something.

My husband follows me in the kayak....he is my hero!

One thing to add, check your local Tri group(s) to see if they are sponsoring any OWS clinics.  Mine just did one last weekend and will have another next month.