if it's spectra it won't snap back at someone in the boat. I've had lines that were knotted break and they just kind drop. there's obviously a little recoil, but nowhere near enough to even remotely hurt someone. However, if it's not a wakeboarding mainline, then yeah, it could snap.
and I've tried with repairing a broken line by tying it together, I have yet to figure out an appropriate knot, as they consistently either come out or break again in the new tied off spot.