Hmmm, marines...
They can be quite useful, but I like to play maps with a lot of water, so perhaps I find more uses for them then most people.
I find they are quite useful, even later on in the game (late modern say) the trick is to have LOTS on them when assaulting a city, and make sure you soften it up first.
Another thing you might want to try is beefing them up a little, or, beef them up a lot and slot them into a later tech. I just finished a game where I had them set to 16/10/1, and it made naval assaults a lot more interesting, instead of landing modern armour next to the city I want to capture, I would capture it with the marines (after a generous naval bombardment of course) and then store my tanks in the city. Even so, I think 16/10/1 is a little to powerful, historically such assaults were very costly to the attacker, so maybe 14/8/1 would be better, or perhaps reducing the cost to allow faster production would better balance it.
Regardless, the marine is no where near as useless as the paratrooper, who Im currently trying to tweak (increase in power, to 16/10/1, increase in cost to that of a modern armour, as paratroopers were usually elite troops, and an increase in operational range).
Just try playing around with the editor until you find something that is suitable.