I whole heartedly agree with DaveR ......... Make your own!!
As Silver Duck points our, watch the "stretch" factor, and also consider the tensile strength (Load limit). The latter probably a little more important than the former.
I ran across this for you to consider -- "Floating Dinghy Tow Rope" at West Marine. If the link doesn't come through simply search the above at their site.
http://www.westmarine.com/webapp/wcs...0&classNum=133