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Under IRC section 162, expenses are deductible when they are "ordinary and necessary" for conducting a trade or business. And while advertising might be considered a legitimate expense for many businesses, deducting the cost of insurance and maintenance on a yacht merely because you fly a pennant with a medical emblem would hardly be considered to be ordinary and necessary.
There actually was a case in the US Tax Court where a CPA/attorney with a tax practice tried to write off his yacht. He flew a red, white, and blue pennant with "1040" on it. His goal was to provoke inquiries and promote business by giving him contact with potential clients in yachting circles. The court held that the yacht expenses were not deductible as ordinary and necessary expenses of carrying on a business. (Henry v. CIR, 36 TC 879 (1961).)
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