Since asking the question some time back, I've learned a lot about shipboard VPN. The success or lack thereof using any VPN is going to depend on 1) where in the world you are using it and 2) on which ship.
Review the many online descriptions on the latitudes at which MN works best and those latitudes where it falls on its face. Outside of MEO coverage, it's difficult (I've often found it to be impossible) to have a workable signal without VPN inserted into the mix. With VPN, you'll likely get nothing.
Within MEO coverage, it will largely depend on the ship. For example, my experience has been that the wifi signal on the CB is generally very good while the wifi on the Emerald is "the worst wifi at sea".
As @Steelers36has so accurately summarized, YMMV.