A small, under 24', tow-able boat small enough to launch single-handed might work quite well. Something with a retractable fin and flip up rudder, so they move out of the way if you ground out, sounds good. You could have a small outboard and sail. That would give you a live aboard capacity of perhaps a week for two or three people as long as everyone was on a first name basis.
If your plan was more along the lines of emergency escape and getting away from a local emergency then a much smaller boat might work. Anything but the smallest boat will give you many times the carrying capacity of what a small group could carry on their backs.
If the emergency has to do with rising water a boat might be much better than any SUV or truck. Of course if that water is going to be moving fast, like near a dam that fails or tsunami, you need some way to avoid being swept along and bashed to bits.