Sit on tops have advantages and disadvantages. You will definitely get wet, and so will your dog, so you will need appropriate, semi - dry clothing which is expensive. The British climate means that you can't use a sit on top for a lot of the year. The centre of gravity is high, so you will fall off, evan though they are usually quite beamy. They are easy to get back onto. I think your dog might struggle with staying in the boat.
I would suggest a sea kayak if you can afford one - they are drier and more seaworthy.
A folder is worth thinking about - I use a Feathercraft Kahuna, but the over £2000 cost is a heavy investment. The plus side is that you can use it all year round.
Have a look at Knoydart kayaking Systems:
http://www.knoydart.co.uk
Take a look at the Pakboats Puffin 2
Good luck.
Enjoy.
