Whats the difference between a

I looked on Lloyds register, but the insurers don't seem to care what it's called particularly, only that it's seaworthy.

Some benchmark figures are 60m long, 500 tonnes, intended for sea use, not coastal or inland waterways. Carrying passengers or freight is not important.

There is no formal definition.

Thats what I thought, that there was no formal definition.

TheCatWoman said:

Thats what I thought, that there was no formal definition

There is a legal definition:

https://opil.ouplaw.com/display/10.1093/law:epil/9780199231690/law-9780199231690-e1197

This points out the differnt standards used for the definition and the range of definitions of what a ship isn't.

As you point out there is no definitive or absolute definition as there have been many competing standards used through the ages.

In general a ship is a sea going vessel (I believe this to mean it can be capable of long voyages over the sea). Everything else is just a boat (self propelled) or craft (pushed / pulled).

There is lots of greyness in the definitions.

In Latin it would be a navis longa.

In Latin it would be a navis longa.