Hi Cliff
When I saw Southern Right Whale off the Valdes Peninsular in Argentina I saw them doing this on more than one occasion. They appear to rest in a vertical position but instead of being head up (which I think quite a few species have been seen doing) they go tail up. I wonder if it's their extra buoyancy that means their point of equilibrium leaves them bobbing around with their tails out the water.
Cheers
Peter Howlett