I was paddling with a guy who had a boat that sounds like what you are describing. He called it a river boat, but I don't know what brand it was. It looked like a cross between a white water and a sea kayak. Not very directionally stable but it was quite a bit longer than a white water boat. It seemed to be a good boat for flat water but I am not sure how it would handle wind and waves. It was old, and he told me they used to be quite popular 10 to 15 years ago.