Although VHF benefits from being mounted at the top of the mast to get maximum “line of sight” range, WiFi reception is over much shorter distances and height is not the critical factor.
Most marina hotspots are at roof height and even with your own WiFi antenna at deck level, good reception should be possible. In areas with big tides, a more common situation is where the boat drops below the harbour wall and this acts as a big barrier to the signals getting to the hotspot.
However, every marina is different and finding an antenna location that will give optimum performance in every marina is an impossible task.