I use too swim all mine at the local dog pool till it got way too expensive but I asked the guy who walks round with them the same question, he said their back end starts too lower in the water, but he also said some dogs would only use their front legs! I think it was about 15 min but he did increase it the more I went, I know its easier to see in a clear pool! But hope this helps, may be try ask some one into greys they seemed to be a few their when I have been, cheers Dave