-- Last Updated: Sep-15-12 9:53 AM EST --
Having paddled hundreds of miles in both a Pintail and an Aquanaut, I'd say they have decent stability, but less primary than most. Certainly less than something like a Romany, Explorer, Cetus, Tempest, etc. They have a very slight v-bottom, so there's a slight wiggle when sitting flat. This is only really a concern for a newer paddler though, IMO, and quickly becomes an asset in the form of more intuitive edging and control.
I'll be sorry to see the Aquanaut discontinued. I think it's a really great boat. Fast enough, but easily maneuverable on edge, and in rough conditions. Carries a ton of stuff, but never feels high-volume. I expect I'll keep mine for a very long time.
The Pintail is mostly an interesting piece of paddling history these days, IMO. I spent years in one, and enjoyed it, but there are now boats that fill that niche far better. Modern ocean-play boats are faster, more forgiving, more maneuverable, and carry a load better. The Delphin, for example, does everything that the Pintail could do, but better.