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.