The idea is very simple: A high gain wifi antenna and router like Ubiquiti Bullet M2 is mounted on the boat or camper. Its web interface is used for connecting to distant wifis.
A smartphone receives the GSM cellular network and works as a wifi hotspot in tethering mode. When this network is needed, the only thing to do is calling the Bullet’s web interface and switching to the phone’s network.
GSM Internet
⬇
Cheap phone in tethering mode
⬇ Wifi
Boat Wifi router (switchable between phone or other wifis)
⬇ Wifi
All devices in the local network