Marine version if near the ocean, so you can load charts.
Ability to read the display in sunlight - take it outside of the store if you can and look at it there.
Those are the big ones. Then you learn to use maps and a compass and stick the thing in the day hatch....
If you are looking for something to primarily track speed and distance, you can get wristwatch sized units that do that quite nicely without the size of add-on chart costs of a larger one.
The Kayak Wing
Heel and Pegpads™
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