What kinda rope? Got pics? You could do like YROC and Spork suggest, but the wrong kind of knot will weaken the rating. If it's a synthetic rope like Dyneema it can be spliced and be as strong as new (tho shorter).
A single fisherman is stronger by like 10 percent, but more likely to unravel. I don’t know how your tow rope compares to climbing rope which is what I use knots for. Maybe there is a difference with whatever your tow rope is made out of.
If it where my rope it would be a double fisherman knot with a follow up overhand knot or a triple fisherman knot with long tails (like 4” each).