I think you need to change accountants. Some tend to be way too conservative. Surely he can get very creative with stuff like this, document the usage and go for it. Don't be scare of an audit, you just need proof and documentation that this yacht was used for business, not too hard to do.

Just remember that the IRS cannot tell you how to run your business! If they had their way, they would rather that I drive around in a Hyundai instead of writing my BMW's off.