I carried a Swiss Army knife for many years before purchasing my first Leatherman. I now EDC a Wave. Both have been perfectly for required tasks. For me, a knife has been most critical when on a boat, regardless of size, or when scuba diving.

The most important characteristics of a knife carried for emergency purposes is that it be situated so that it is quickly and easily at hand when needed and that it be adequately sharp. I have about thirty blades ranging from a machete to a SAk Classic and they have all come in handy at times, but something with a blade 4" or so is probably the most versatile.