Sea Slug?

When we studied the itinerary for the tour I noticed that there were no opportunities to potter on the beach or to paddle in the sea, so we arranged these few days with a hired car to do just that.
Today, in the rain, we wandered by the shore near Blenheim, where Mr Flum noticed this creature, which I think is a sea slug, (it's about 6inches long) alive but above the tide line. I hadn't intended to paddle but a large wave soaked me almost up to my knees as I helped it back into the water.
Next time I want to play Dr Dolittle I'll at least roll up my trousers and take my shoes and socks off!