DH and I have Wilderness sit-upon kayaks. Mine is the Tarpon 100 (10 feet long):
http://www.rei.com/product/785482/wi...kayak-20112012
His is the Tarpon 120 (12 feet long and narrower)
http://www.rei.com/product/847396/wi...a-001b2166becc
We have used them on lakes, creeks, and rivers (even some small white water rapids). They are great, but I would get the 120 if I were to get another. It is slightly narrower and easier to paddle. We tried a 14 foot one, but it was a bit long for maneuvering in rocky waters (at least for me). Our dog travels with me and even when she jumps out I have never tipped. The sit-insides are more stable for white water (and lighter to carry), and we may be getting those this summer.
Be sure to shop around, we found an outfitter who sold them for about half of retail. Colors were limited, but price was great.