Don't think you need one on your kayak?  Even for short day trips, you'll be surprised how quickly you can become disoriented, since you're low in the water and don't have long sight-lines.  Following a compass course is good exercise while paddling---you'll go straighter and cover more ground for the same amount of energy.  The Ritchie XP-99 Kayaker Compass is quick to mount on kayak or small boat decks; features a sapphire jewel movement and adjustable bezel.

Removable for security.  Also great on smallcraft;  we've used them on racing dinghies, daysailers, and pocket cruisers. 

Works out-of-the-box on cambered kayak decks, but a mounting block makes for an elegant installation.  Instructions for the mounting block are here, and the compass mounting template is here.