The only legal recourse you would have would be to sue him for contribution for his share of the necessary and reasonable expenses. As the terms imply, he could challenge whether the expenses are either necessary or reasonable.
You could also sue to have the partnership terminated and the property sold. As part of that action, the boat would be appraised and you could be allowed to purchase your partner's share, less his share of the necessary and reasonable expenses.
You could raise both alternatives in the suit.
Chances are that the fact that the suit has been filed will be enough to motivate him to settle with you. Filing the suit could be done in small claims court, most likely, which does not require an attorney and does not cost very much (e.g. around $50 to $100 filing fee). Moreover, the clerk of court provides you with the forms and instructions for filing.
Please let me know, if you have follow-up questions.
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Attorney
Attorney and small business owner with 8 years experience in the general practice of law.