I've got a Cradlepoint MBR1400 for sale. It's a fancy router + wifi base station that also has a couple of USB ports where you can plug in a 4G/LTE modem and get internet throughout the boat. People also refer to these as hotspots.
It additionally has multiple WAN ports where you can connect a wifi externer like a Rogue Wave or Island Time to pick up marina or other wifi and pipe that throughout the boat. The Cradlepoint can automatically detect which WAN connection is available and automatically switch between them. So, for example, if marina wifi is available via a Rogue Wave extender, it will use that to provide internet for the boat, otherwise automatically switch over to the 4G/LTE modem. The Modems typically cost $20/month and use whatever family data plan you have.
Here's a link to the user manual
These sell for $300-$350, and I'm asking $150 plus shipping