Yes, one needs to be able to swim which necessitates that one is strong enough to float in a body of water in case they lose their balance and fall off the kayak. This would mean that they have to be at least a reasonable weight so the boat does not topple over and also so that they fit into the small-sized vessel. Also, they have to be strong enough to move the kayak using the paddles, for which a person needs to have a strong upper body.