Auto compasses have worked fine for me on my kayak for about 20 years (or at least until they fell off, got knocked off, lost their damping liquid or otherwise kinda-broke). The key is to mount them in the "proper" location - as far from the paddler as it can be read while paddling. As others have noted in this forum, mounting them directly in front of the cockpit forces the paddler to look down to read it, contributing to seasickness in bouncy conditions and making it very difficult to paddle and hold a bearing. Mounting them farther from the paddler means one only needs to dip one's eyes to see the heading, making it easier to follow a bearing. Taking a bearing off of distant objects is easier, and the equilibrium of the inner ear (responsible for balance and seasickness) remains undisturbed.

With the compass in front of me, I see and read only the bearing under the lubber line which is facing me as designed. Yes, if one "reads" the compass from the top or front/bow side (not to mentions the port and starboard sides - not a lubber line in sight anywhere!) one may see the rest of the compass dial. I have not found this to be a problem. Get a compass that has graduations at least every 5 degrees (you can estimate well-enough from there to a single degree). You will need a protractor compass (silva, suunto, brunton, etc.) or other navigation protractor devices to get your bearings from charts and maps.

Since my kayak isn't made from solid metal, I usually remove the compensating gadgets the compass manufacturer built into the compass to adjust for the metal masses of vehicles. I also try not to have my steel-ammo-box camera dry-box close to my compass if readings are more critical. Navy ships also have to adjust for the effect their steel hulls and structures have on their compasses.

: You want to definitely avoid any compass made for automotive use. Their
: readings are backwards. Trying to use them in conjunction with a map and a
: handheld compass will send you off in the worng direction.

: a handheld compass, and a proper nautical compass that is deck mounted, are
: designed to be read from the top, while automotive compasses are designed
: to be read from the back. While this makes sense for a car driver who has
: one of these things on his or her dash, and is not comparing it to another
: compass, the readings are opposite what you get with a "normal"
: compass.

: let me elaborate. Suppose you are in, or next to, an automobile that is
: heading north. Since the manufacturers of these things assume that the
: compass will be mounted in front of the driver, the automotive compass
: will have a floating ball with the letter N on it, facing the driver. But
: to get this display, that N has to be on the SOUTH side of the compass. To
: make it more confusing, the letter on the NORTH side of the compass --
: plainly visible through the front window of the car, or from above, is a
: big "S".