It doesnít matter where the boat is registered but if you have a valid lien naming the boat you can arrest the boat in the United States. If you want to chase the boat outside of the United States and the work was done in RI you need to start an action in the country where the boat is. A lien is not portable.
On the other hand if you get a judgment naming the owner and he has propriety in another state or country you can prefect that lien and seize assets that are in his name and it doesnít matter where the assets are. The court will order the seizure and sale of the assets.
I think you will need an attorney unless you have a lot of patience and donít mind doing a lot of research to get the paperwork right. Itís not straight forward and he can bond out of it if you arrest the boat. One action is in Federal court and the other might be in State court with a significant different in cost depending on how you do it.
Good luck and all the best,
I am not an attorney and donít play one on TV. I owned a marine business for 35 years and have been involved in salvage, commercial repair work and litigation involving both. You need an attorney and depending on how you want to proceed you may need a Admiralty Attorney or simply a civil attorney.