If my understanding of materials is correct, it depends on what the tarp is made of and how well it's impregnated with repellant if a material that is porous.

Once again, if my understanding is correct, pressure on silicon impregnated nylon (for example) will allow water to bleed through where the line is if the tarp is draped over the line.

That may be only if the line is wet though.

I could be wrong though... Suddenly I'm not sure where I heard that stuff. HAHAHA

On a plastic tarp, it may not matter unless water travels down the line and drips on you.