opposed amphibious invasions

[Disintegration]

Something that hasn't ever come up playing vs. the AI but I definitely need to know before playing a human opponent: what's the score with amphibious invasions against defended hexes? As the CP, is it necessary to put a good, well-entrenched stack in every port or will a single depleted infantry corps do to keep the Royal Marines out? I'm not looking for numbers to crunch or anything, just some basic idea of how this is handled, and how most folks handle garrisoning rear-area ports.

Also, I notice playing the TE that not every coastal city is a permitted destination for amphib movement. Is there a list of which coastal cities are actually ports, or do I need to just start a throwaway HTH game (playing both sides) and put ships on amphib in every sea area and copy down the cities that are available?

[SMK-at-work]

You can only amphib invade with 1 corps IIRC, and if it does not push the enemy back then it is destroyed. 

All coastal cities are ports - however you cannot amphib invade all of them - any that have a CA or larger naval unit based in them are out, as are US and Canada IIRC.