Rod, yes keeping the antenna vswr low on a ship that is going up and down with the tide is a bit of a mission particularly when its moored alongside a fixed jetty. The Q on the antenna will be particularly high with a near perfect seawater ground plane of very low impedance. As far as the equivalent circuit is concerned you have radiation resistance, loading coil resistance and ground plane resistance all in series. When all of these are low, the Q is high. The upside is that short antennas can be quite efficient, the downside is that its extremely narrow BW and not very stable. That said it does seem to be working OK assuming it is the short vert in use and not the inverted L. I think the latter would be my choice as it's there.