I've just used pretty standard dock lines. Relatively soft hand so they don't scratch things up, can be found in many lengths with a pre-spliced eye or custom made by splicing an eye yourself (which is really not too hard). Only had reason to tow an OC-6 once and it was a very flat day (even for a lake), but ran one line from out at the ama and the other around the front wae wa'a to a single tow line, probably did two bowlines there to join them but I don't really remember. Worked well except for the boat wake tossing the canoe around a little bit (it was a big boat).