Water doesn't behave the same as air. For a start it doesn't compress. Just ask anyone who has bent conrods in a engine that ingested water.
Paddling provides constant support on both sides. That alone is enough to make the kayak more stable.

: Being an Aircraft Mechanic by trade I can add a bit of science to the
: equation; The kayak is basically a wedge and as that wedge move through
: the water it compresses the water around it. the faster the kayak moves
: the more the water is compressed. It's this compressed water that is
: actually balancing or stabilizing the hull by adding equal pressure on
: each side. Think of an arrow or the old pointy paper airplane, these are
: much more stable in the air the faster they fly. A faster jet will have
: narrower more wedge shaped wings for speed but would stall (become
: unstable) at too slow a speed.