Onboard, i've got rigging knives, swiss army knives, dive knifes, scaling knives, leatherman tools... but the knife that always seems to be at hand is this cheapo:
I got it as a Christmas gift, just a stocking stuffer, but it is a better knife than it has any right to be.
Lockback knife blade on one end, retractable utility blade on the other, opens easily with one hand. The only drawback, which wasn't that hard to rectify, is there was no lanyard ring. two minutes on the drill press and a flag clip solved that problem.
I'm not a big fan of spring knives, simply because i have lost two overboard, both times when I tried to open them with greasy hands. Sprung right out of my hand and gone.