The only way I can think of is to make a special type of land terrain that looks like water, and make it impassible to wheeled units. Than make every unit except for one "Wheeled" (you'd also have to remove the wheeled flag for jungle and mountains).
Yes, but the Ai will not be able to use it. Make a unit air, but do not give it the immobile flag, and if there is anything else, I don't remember it.... but I do remember making hovercraft for my mod about 2 years ago. I pulled them due to AI being completley unable to use them.
Air units can't defend themselves from land/sea units, so your unit would be practically useless unless you want to escort it everywhere (which would be hard, considering you would need both land and sea units).
An alternate is to make it a land unit and give it a small operarational range and whatever air missions you want it to have, so that it could bombard ships, scout out sea tiles, etc.
So, Finn, you may want to reconsider your lottery request... a LOTM unit instead perhaps...
In many ways, I like the Civ3 helicopter. Helicopters can't really exist away from base indefinately. Here is what I would have done to improve it, though.
1. Allow it to pick up units (simple suggestion to give a helicopter more of a use).
2. Allow the creation of Attack Helicopters using the C3C "Stealth Attack" feature. Helicopters can fly over the heads of enemy troops and be much more precise in their targets. They were dispised by the Vietnamese army because they could shoot from above, negating what defenses they created. Also, in Operation Iraqi Freedom, Apaches were used at one point to minimize non-combatant casualties. For the negative side, they should be rediculously easy to shoot down for anyone who has some kind of AA).
Definitely this is another one of those many limitations Civ imposes on us. It is frustrating knowing what you want to do, and knowing that what you want to do is realistic, but the game's design is very restrictive. I hope Civ4 "fixes" some of these problems. For now, the full mod-support they are promising gives me hope.
I agree wholeheartedly, Louis XXIV, on helicopters picking up units - how can you have air cavalry without that ability? In Vietnam, that was the original reason for their deployment.