I'm a bit baffled by the Cottonclad/Gunboat rules. I understand the bits about 0NSP etc. But do they always have to start at an Ocean Port precisely at the opening of a river and do an Amphibious Assault up that river? If so, do the Amphibious-Assaulting troops have to start in that specific river-end Ocean Port?
The italicized preface and play note are not the rules. They illustrate some of what is possible under the rules, but they are not the rules themselves.
Other things I don't understand include: whether they allowed to skirt along coastlines etc then go up a river; whether they can start in a River Port in a river; whether they can go down a river; why it allows an attack from an Ocean Port to a River Port (I thought that wasn't allowed); whether they can 'link up' somehow with movement over Oceans.
Basically ... what do they do?!
Think of them as 0 NSP Naval Units that automatically accompany your SPs that are performing an Amphibious Assault for the whole Amphibious Assault move. They automatically appear when you activate the Amphibious Assault with the Gunboats or Cottonclads and automatically disappear after the conclusion of any combat that occurs as a result of the Amphibious Assault. Otherwise, all the normal River AA or Ocean AA rules apply. Note that the gunboats or cottonclads are only accompanying the AA. They are not carrying the SPs. So, if you are going up a River, the normal River AA rules apply to the move unless you are performing an AA in the tidal portion of the Mississippi (on or downstream of Vicksburg).