Yes, all must be connected to the same net. I thought we were talking about a net inside your boat and wasn't aware of the "marina". I better describe my net;
I've a 12 V wifi router inside my cabin
normally not connected to any Internet
or external stuff like a marina router. The router act as an DHCP, sharing IP addresses, and its IP address is 192.168.10.1. (Chose whatever beginning with 192.168. . .) Every other of my gadgets will get an IP address in the segment 192.168.10.(2-253)
This is my local boat net where the UDP "broadcast" is sent and received independent of other nets outside the boat.
If I would like external connection to for example Internet the same router can be used with a USB 3G-dongle or connected to another router set up as an access point. Maybe to a marina router.
Answer 1. Yes
Answer 2. No
Sorry if I was (or still are?) unclear and presumed things obvious for me and didn't thought of that all are not electronic geeks.
But I see you are on your way to find out things that will work for you.