Further to my reply to the properties of water post "Does Water Have a Fourth Phase...' I've done a little research and found a sub-species that has evolved to be even more efficient at travelling across water. It has four pairs of legs, with the hind two used for propulsion - the fore two for balance. Although bulky in appearance it is light in actuality, with air spaces between muscles and ligaments, large lungs and a bone structure similar to the lacunae of bird skeletons. It's rather like being foam filled.
Videos of it in action can be seen on youTube.