What items do you need on a boat to make it legal?

From none whatsoever to a list that would require several books.

To use it in some places then that jurisdictions may demand certain things, especially if you are carrying other passengers, as will insurance companies if you intend to insure it.

The requirements vary from nothing at all to a huge list for licensed sea-going passenger vessels.

These may include anything from life jackets and an auxiliary lighting source a set amount of flares and signalling equipment.

You will need to check the legislation where you are for the vessel you wish to use.