I use a mix of some similar things cited here already:
Peplink MAX Transit router - this provides both 2.4Ghz and 5Ghz WiFi networks inside the boat, and has tons of features to
manage bandwidth usage when you're out and about.
It also has dual SIM slots - I use T-Mobile and Verizon unlimited SIMs and can flip back and forth easily between these. Verizon in the US, and T-Mobile in Canada.
To get a good cellular signal, I use a weBoost Drive amplifier and an external marine antenna. This has pulled in LTE/3G signals in places where I never thought I could get coverage at all. The internal antenna is a few inches away from the cellular antennas on the Peplink, and is also in the center of the salon, so handheld phones get signal too.
For WiFi boosting, I use the
MikroTik Groove connected to the WAN port of the Peplink. I use it a lot at my home marina where there are Comcast WiFi open points I can snag, but in many other locations it isn't that useful, as others have said. Marina WiFi systems are unreliable in many cases, or are just too far away. I do recommend the Groove though, as it handles both 2.4Ghz and 5Ghz networks, and if you can get 5Ghz, it is usually quite reliable.
I
wrote about most of this for my previous boat, but definitely need to update it for the new one as some minor items have changed.