Bought the Navionics on the weekend and played with it a bit. You're right, you can't go wrong for ten bucks for the entire Great Lakes including the St. Lawrence. The GPS in the 3GS is bang on with placement, even put me on the right part of the slip where I was sitting.
Here's a pic.
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I'm sure the iNavX one has more features but I figure I would try this one out first.
SVlagniappe, thanks for pointing this one out to me and saving me some money.