A point of terminology, which you've probably already absorbed: A sailor will probably give you a blank look if you ask about declination, and a land-based expert has given me the same when I've asked about variation; they are, of course, the same thing, but we sailors can't stand landlubber terminology and therefore insist on using our own!

Marine compasses traditionally can't be adjusted for variation, only for deviation, and as another poster mentioned, mariners traditionally navigate by magnetic north as marked on marine charts (including NOAA's). Like another poster, I'm a longtime boater new to kayaking, and I haven't yet firmly decided how I want to fit out my boat, but I'm leaning toward a handheld GPS and probably a handheld compass, possibly with a detachable either-or mount. Have to admit, though, there's a nice secure feel to having a compass sitting securely on deck, and for your application (variation about 20 degrees--I'm used to