I am going to tag this under admiralty law to bring in some more opinions.
An imaginative (admiralty law) guess on my part would be to have your son agree to let you exterminate the rats. Then you can demand he pay for the extermination or have the boat arrested. Your son could then either pay you the amount owed and the rat problem vanishes, or the court will sell the boat at auction. After the sale, the boat no longer belongs to your son, and he could do nothing to stop the new owner from removing it from the property.
Of course, you could just hire an attorney to demand that he remove the boat or it will be hauled off your property. The effectiveness of that strategy would depend on whether he lives there, pays rent, storage fees, etc.
So really what I am saying is that you should hunt down a few local attorneys who offer free consultations, give them the specifics we attorneys tend to love, and see what they say.
Best of luck.
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