I definitely can't offer anything as technical as some of the responses, however, the post intrigued me so i pondered it a bit as I got in some distance over the weekend. My boat is 450mm wide at the beam and 6.7M long, so it definitely falls into the the long&skinny category.

Yep, it feels a bit tippy particularly when a high speed ferry zips past and I do feel more control when I am paddling but I'm with Allan and Toni on this one, I beleive that the engine is responsible for the stability. the paddling action and the paddle itself combine to provide stability.
I am almost certain that if i sat in my craft with my paddle out of the water and I was towed along by another boat in perfect conditions I would not find my stability increasing. More likely I would find myself practising re-entries.

This is a bit of a generalisation, but when I see somebody paddling a skinny boat such as K1, Tk1, surfski, they all appear to have a decent forward stroke which must also contribute to stabilising the craft. i.e good leg drive, torso rotation, centralised body, etc.

My vote is for the engine. Even the clapped out donk that drives my boats.