I can see why you might think this is an ichthyosaur paddle but the photograph looks a little shiny? Are you sure it is bone? I think I would need to see this up close to be sure - it would be good to compare to the specimens in the reptile gallery of the Natural History Museum, London. Please bring it in to the Angela Marmont Centre and I would be happy to look more closer at your specimen!
Hope this is helpful - sorry not to be definite, it is so difficult from images alone.