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Theartist: When I was moored at Brundall I connected via a local BT hotspot because I am with BT Broadband. It worked perfectly for me. I don't know how you connect with other broadband providers or what the availability is like. You need to qualify and clarify your question.
A range extender may well help but, you would still need to know the password of any router you try to connect to unless you are with BT or have a provider with a similar arrangement when you can then use your own account name and password. Open wifi just doesn't exist any more except when it is a public wifi network (and there isn't one in Brundall). You could always go to the Ferry House at Surlingham and ask to use their wifi - I have done when staying there overnight.
I don't use anybody's WiFi, I just use my phone as a mobile hotspot. I link my tablet to it and my wife links her phone to it. Data is a tad slow in one or two remote locations, but works fine all over the north and south rivers.
Yes, I've done that too - its much easier to use a tablet or laptop than a phone. But the OP was asking about wifi, which is password protected and has a limited range. I don't understand what he is expecting (and perhaps he doesn't either as you can't just piggy-back onto the nearest wifi router).