Open water swim "assist"

When I was training last summer for my first set of triathlons, I did some swimming in a nearby lake. There I saw someone swimming while pulling a large inner tube behind him. I did not have the opportunity to talk with the swimmer, so did not learn anything about what looked like a good idea. It seems this would work as a safety device for a tired swimmer and/or someone who could only enlist a "sit on the beach and watch me" friend for an assistant in open water swimming. My first thought is that the tube looked awfully big, and cumbersome. so I wonder if there is a smaller device that would work, or is the size unimportant? Next -- what does one use for the rope/strap to pull with? And where does one find this thingie? And of course, has anyone actually used something like this and what has been your experience? And are there any "tricks" one should know about using such a device? Thanks, Nancy Graham