When first thinking about this issue I was of the agreement that we should simply do a few things:
1) Free Amph Upgrade (let's face it, heli's can attack from water)
2) Use of rails, but not roads
3) Ability to be transported by Carriers
and that would be enough, however, in thinking about how heli's are actually used, I think an increase in movement is also warrented, they are often used to transport units well behind enemy lines. So more than a 4 movement should be included, perhaps as high as ten. Then I remember all the reports of them being shot down, so I think, hey, if you move over or next to an enemy unit, allow them the chance to attack. After all this I think it would make sense, but wait, I remember that this is called a gunship. And this description sounds closer to a transport heli, sooo, I'll stick with recommending the first three suggestions, and maybe give me a new unit... there should be more units in the game anyways.
On the note of flying over water, I'd say to give it to them, but in reality they need to land at the end of their turn, on land or transport/carrier. So other than moving them to a ship, with a current movement of four, I can move out two units and move back two, doesn't really help to be able to fly over water, so I don't care about that feature, but one things for sure, they are definately underpowered as is