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

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