So, if a rig with say 100+ people on it requires a crew change every two weeks, as is often the case in many parts of the world, and over 15+ rigs/vessels/fpso's/tankers nearby all requiring the same, then is one boat going to accomplish this? Nope. It's going to need a fleet. Then what happens when the sea state goes above 3m? AND, the distance between the furthest rigs is 100miles+? Can a boat do this in a day? Nope.
Some bases flying offshore can do up to 19 flights a day, transferring hundreds of personnel A DAY. I wouldnt worry about losing your offshore flying to a boat anytime soon.