I'm in London at the moment, as are many boaters. It's a bit overcrowded here.

For canal boating, absolutely. Maybe 20-30k for a cheaper boat. Can moor for free. Coal and fuel maybe £100 a month each, depending on how much you travel/if you have solar etc. You do need to black your hull every few years, but they’re generally sturdy and don’t need much more than that.

Yachting, I have no idea.

Yes, I've been very curious about real world experience of passports in this lifestyle.

Yes, usually, but it depends on the country. On top of normal travel visa you also need to make sure your boat is documented from the country of origin and often are required to pay some form of tenporary import fee for the vessel as well. Different places have different rules, some (such as French Polynesia) let you extend your visa up to a year or more if you visit by boat but then you will need to sail elsewhere before you are able to come back.

I suppose you don’t have to declare that you are in a country you visit by boat, but the consequences of not doing so could be extreme if you are unlucky and caught by the local authorities

Near shore, LTE hotspots and/or Starlink RV are feasible options (or, in a marina, there's often marina and/or municipal WiFi around)

Canals, I use 4G for everything. Do market research and find the best provider. It's difficult but not impossible to web dev.

Diesel Engine. A bit diesel HR2 that I had to take apart once!