any advice on handheld gps units for lake mapping

Lowrance H2OC

I agree with the H2OC and the H2O.

If you get a Magellan Sports Trac - invest in a battery company, 'cause they eat 'em  :'(

All you need are rechargable AA batteries.

When I bought my first GPS unit, I went through 8 AA's in less than a week.  Naturally, I was pretty PO'ed.  Went out and got some rechargable batteries and haven't had a problem since.

I use them in everything.  GPS, digital cameras, portable depth finder, TV remotes, clocks, etc.  You'll pay more for the rechargable batts initially, but the savings in the long run far outweighs that higher investment.

My H2O came with a cigarette lighter plug.  I went to Radio Shack and bought female half of the plug, ran a couple of six foot leads of it and attached aligator clips.  When I'm on the water, I simply attach the leads to my trolling motor battery.  No more worries about AA's.