depending on which way the boat docks and its size will determine how many access point you will need.
I would recommend mounting an AP on each side of the boat to cover the above scenerio, and having the Root access point mounted in an ideal location on the shore.
I feel you will be better off with Mesh unless your in the US and can use the 1400 bridge APs, the the Mesh APs they will auto discover themselves and will determine the best root back the the RAP.
Hope this helps
To have a single AP and multiple antennas will work for short distances however the moment to move over 10 meter from the AP with an antenna extention cable you will start to loss alot of signal in the extention cable.