I have two pair which I use to fly fish in really cold water. I had worn them with the Simms Guide weight waders and never got cold going to and from the rivers and wading in them.
They worked so well, that I wore them to the coast with a pair of Crocs to fly fish. When the first wave went over the top, I was surprised at how wet they got and how cold the January water was.
Now, I wear them in non winter months to the coast with the Crocs with a tread as they keep most of the sand, grit and whatever off the skin from my ankles to my feet.
I don't have personal experience with the SealSkinz socks but have heard they are great as long as the water level does not go above the top of the socks. Once water goes over the top they are no longer waterproof and water then gets trapped in the sock for the rest of your outing.
That said, they may be fine for simply hanging your feet over the side as long as you don't go too deep.