I have studied boat lengths some in technical and practical terms and short boats are slow. Slow, meaning you get less speed for the amount of effort you expend. A boat with a longer waterline will have a better speed with the same amount of effort. There is going to be minimal storage in a boat as short as you are talking about and if the water gets rough (and it happens) the short boats tend to look like a kids rocking horse in the waves!

When it come to boat designs there are few things that are clear cut but short waterlines (length of the boat in the water) are slow. I have seen lots of people show up at group paddles with their short boats and what happens is always the same. They struggle to keep up and we have slowed down so as not to run off and leave them. No one leaves happy! They either give up kayaking or soon trade up to a real kayak. We always try to get them to try one of our boats so they see the dramatic difference.

Short boats have to be very wide to be stable. So you end up with a lot of 'wetted surface'. This is the area of the boat under the water. This area leads to a lot of friction. Longer boats can be narrower and have less wetted surface and a better shape. There is a lot more involved in hull designs but I never recommend a short boat for a flat water paddler.

What length you need is a little subjective but I have found that at around 13-13.5 feet of waterline which is around a 15 foot long boat is the sweet spot, resistance wise. That will give you respectable performance, it will be more comfortable when the water conditions get a little choppy and it provide you with a lot more storage. Based one what you have said I am with the others and recommend you keep looking.