Service charges are collected by the management and generally distributed amongst the staff. Sometimes the server gets as little as 7% of the 15-18% service charge.

If you want the person serving you to be compensated 20% then you'd need to add on top of the service charge.

There is a very popular place on the North Shore of Tortola that requires servers to turn in all tips (even cash discreetly given directly to them) and then they distribute at their discretion but only to a maximum of $10/hr. I think that policy is terrible and very unmotivating for employees.