Yes a boat is a capital asset under the internal revenue code, regardless of whether its use is personal or business. Therefore the same rule
applies as to computing the gain.
Its rare that you can sell a boat for a lot more than you paid for it, though. I wonder how many people remember to claim the gain on their tax return.
also need to distinguish between maintenance
(which does not adjust your basis) and capital improvements (which do).
The CPA wife was not asked the right question. Then again, I am not a CPA, just studied tax at New York
University Law School
and am CFO of 4 public companies.
You might wish to review IRS Publication 544, Sales and Other Dispositions of Assets.