Seatbelts are very hard to cut because they are so tightly
woven (is woven a word? - weaved?). Your knife has to be extremly sharp, so dont use it for anything else, and have it
sharpened again if you are ever
unlucky enough to actually have to use it (
lightning can strike twice, but you'd have to bloody unlucky to
drive into a lake twice).
You can buy a combination
tool that has a section that is designed to
cut seat-belts easily. The tool is shaped like a
hammer, and has a pointed metal end which is for
breaking the windows. Not sure where you get them - probably at a
hardware store or
car accessory shop.