I'm pretty sure you can register a vessel in any state/province/sub-region of a country but when it comes to flagging a vessel, choose your country and go with it, for what ever reason. There are a variety of terms that each country might use.
For example a boat can be registered in any US state but the only national 'Registry' is the USCG 'Documented Vessel", very different from a US state registration.
, I belive that you can register in a province but you can also pursue a Federal Canada
DOT registration which is a National certification
, similar to USCG Documentation
in the US. Gordon May is probably a reliable source for that info.
There are many 'Flags of Convenience' from the Cayman Islands
to Liberia to Jersey which all have tax or other advantages and the price
of entry varies.
Unless you are trying to set up shell corporations or hide out from tax collectors, there normally aren't too many advantages to leaping through the hoops. i've done the documentation/registration shuffle for a number of clients who because of local country laws were unable to have their vessel remain flagged to a particular nation but unless there is an overriding reason, it really isn't worth the trouble.
Trying to sell a vessel with multiple flags
brings with it a whole other host of difficulties IMO. Capt Phil