What is the difference between a boat and a ship?

Finding cut and dry, black or white or definitive rules that without doubt define a vessel as a boat or a ship is not easy. Rule of thumb indicates that a ship is an ocean-going vessel that travels long distances out on the high seas often to other countries or continents, has a larger crew, larger cargo capacity, more technology and heavier-duty equipment than a boat. But that much is obvious and doesn't unblur the threshold where a boat becomes a ship, especially as the demands of modern industry and tourism has led to the production of numerous larger bespoke boats and smaller ships with a mixture of technologies and staffing requirements onboard not to mention where older ships with sails fit into all of this! But maybe that's the point, maybe we don't build ships or boats anymore just vessels that meet a criterion one of which may or may not be deep-sea capabilities.

What can't be disputed is that cruise ships, naval ships, container ships and tall ships are all ships, we know this because they have the word ship in their title, likewise, canal boats, tug boats and rowing boats are definitely boats, but what about tankers and ferries? They can be found operating on both the high seas as well as rivers. This could be taken as further evidence that a vessel's usage has more to do with its categorisation than physical design.

The most compelling factoid out there tackling the subject suggests that a ship is generally a vessel of more than 500 tonnes, but even then, a short internet search will produce a long list of vessels over that weight that have never seen salt water so contradicts other defining criteria.

It looks like the mystery cannot be definitively solved by ticking boxes alone as there will always be exceptions to the rule, even the New Collins Dictionary defines a ship as a boat and a boat as a ship so it would appear that each vessel is categorised on a case by case basis by its manufacturer or operator although it's fair to say, a sizable ocean-going vessel is likely to be a ship, probably!