First of all my name is Michael Newbury and I live in London, England but foremost I am a Marine Surveyor. I would suggest that you try to find out if he is a member of a yacht club. If he is a member they will most likely have details of where he berths the vessel. Next you should contact those companies that provide yacht insurance. Do you know the name of his insurers. They have got to have details of where the vessel is normally berthed and sea areas it is allowed to operate in. You should also find out if he has a GPS system - normal equipment these days - and a radio call sign. If the vessel is equipped with a radio and can go deep sea it will need to have radio identification - call sign.
He is not permitted to take a juvenile outside coastal waters.
Ask the USCG if they have a register of yachts and what are the criteria regarding registration. His sail boat - we call them yachts - may be of a size that requires him to register it with the USCG. In which case the last time the yacht entered a harbour or dock could be on record with USCG. If it is on record you may well be able to get the yachts last location.
Don't talk to a lawyer unless you have absolute evidence of a wrong doing by your former partner.