My opinion:

If you are going to ride on unpaved boat yard lots you need 20" wheels. 16" will probably not work. If you will ride on pavement 16" is Ok. Strida 3 is a 16" bike that could work for you.

As to gearing you could chose:
1) single speed. Least expensive and best on level ground.

2) Derailer bike. The bikes you suggested use derailers. derailers require adjustment and can be damaged. This might be a pain on the boat, on the other hand boat yards are filled with handymen.

3) Internally geared hubs. More expensive but require much less maintenance than derailer but are more expenive. consider the Dahon Vitesse D7 HG.