Question: swim dock and lines in the water

[Frank]

We have a swim raft and a trampoline, both about 50' out from our main dock. Sometimes we see a boat or a PWC go between our dock and the two outlying platforms... a dangerous thing to do in my opinion, but I suppose as long as they are going headway speed and there are no swimmers there, they aren't breaking any laws. But, here is my question: my kids sometimes run a line from the dock out to the tramp or the swim raft. They use it to pull themselves out there while on a float or kayak. And, being kids, they don't always put the line away. If a boat should cruise in close to my dock and suck that line up into its prop, am I responsible for the damage to the boat?

Thanks,
Frank