26 USC 6718 - Failure to display tax registration on vessels

(a) Failure to display registration 
Every operator of a vessel who fails to display proof of registration pursuant to section 4101 (a)(3) shall pay a penalty of $500 for each such failure. With respect to any vessel, only one penalty shall be imposed by this section during any calendar month.
(b) Multiple violations 
In determining the penalty under subsection (a) on any person, subsection (a) shall be applied by increasing the amount in subsection (a) by the product of such amount and the aggregate number of penalties (if any) imposed with respect to prior months by this section on such person (or a related person or any predecessor of such person or related person).
(c) Reasonable cause exception 
No penalty shall be imposed under this section with respect to any failure if it is shown that such failure is due to reasonable cause.