I use a Garmin 60csx for boating and hunting, hiking, geocaching, roadtrips, etc. It's been around a few years but I consider it the "gold standard" of handheld units, especially for its accuracy. Haven't checked prices on eBay or Amazon lately but my guess is you could get one for around $200.
Check out this site for tips on using it:
http://www.asiteaboutnothing.net/g_60csx.html
You can also download a navigational depth map of Lake Winni for $10 here:
https://buy.garmin.com/shop/shop.do?cID=254&pID=36400
I have used this map and it is pretty decent.