At longer distance, the vertical beam angle is less of an issue. It is really only at shorter distances that it is a significant issue. The further you are from the WiFi hotspot, the more height the vertical beam angle will cover.
That's true if your antenna is at a different height that the shore-side antenna. If there are surrounding cliffs, for instance with houses on top.
That's less true is you are purely concerned about pitching* at anchor. Then extra beamwidth will really help you. I find that our boat doesn't pitch that much usually. (50' boat, 10 foot water depth, 3 miles of fetch, monohull.) But the 30 foot boats around us have pitched up and down 20 degrees from horizontal in a blow. (Remember Mainsails videos of anchorages up there?) That's not the normal conditions at the anchorage, but it is a time when you would want connectivity to see where the storm is going.
As another thought on this, the TCP/IP protocols have some provision for lost packets, IIRC, so if you have the occasional extra pitch, you may still have service -- it simply retransmits that part of the packet that was lost.
Boats usally don't rock (actually roll) at a mooring/anchor as much as they simply pitch with the approching wind swell. That is, unless you are exposed to swells at your anchorage, in which case you may actually need a big verticle beamwidth to accomodate more intense rolling.
I believe that for 95% of what I need, a high-gain, omni-directional antenna works best. 12 db is great. I'd go even higher (and narrower beamwidth) if I could.