... In the USA it is ok to use a marine or commercial rig to transmit on ham frequencies, but not legal to use a ham rig on marine or commercial frequencies (except in emergencies, etc., etc.).
Actually, it is illegal to use a ham rig on the marine or commercial or aircraft bands virtually anywhere
in the world, not just in the U.S. The UK, Canada, and the European Union countries all have published standards, as do a number of other countries. See, e.g., Air Waves - Issue 4
(scroll to "News From the MCA").
It's really pretty simple to understand:
If you're a licensed ham
, you can use just about any radio equipment in existence -- including your own homebuilt sets -- on the ham bands for which you are licensed. And, only on those bands. The idea is that as a licensed ham you have demonstrated some technical knowledge, and you are responsible for your emissions and must see that they don't cause harmful interference with other stations.
If you're going to operate on the marine SSB bands
, you must have: (1) a marine operator's license -- at least the Restricted one; (2) the radio must be type-accepted for use on the marine bands; and (3) you must have a marine station license, covering the use of that equipment.
That a ham radio can be made to operate on frequencies used by other services -- marine, commercial, aircraft, etc. -- is irrelevant as far as the regulations go. With very few exceptions (perhaps MARS/CAP operation, with proper authorization and, of course, in a bona fide life-threatening emergency situation), such operation is illegal. Which is not to say that a number of boats don't do it more or less routinely.