Why not just do it while the boat is at the dock before you leave?
If possible, sure. I would ask the dive crew what they suggest. Obviously, don't just strap on a tank and jump in the water without their permission. For one thing, you'd probably need the boat ladder to be deployed, as there often isn't any sort of ladder into the water from a boat dock. From the boat, at the dock, is exactly how we were requested to do our weight check before the first dive of a dive resort trip I was on.