Marines are great units, well worth building even if you have oil, but if you're looking for them to be the bulk of your army, you're not using them correctly. The best use for them is to terrorize coastal cities. Take them, and hold them long enough for you to get proper garrison troops there, then load the Marines up, sail to the next city, and repeat. Properly promoted, they do well against infantry, and with enough cannon fodder, they can hold their own against mechs, too.
In transports, they have an effective range greater than any other land unit. One of my favorite strategies is using one or two "landing forces" (battleships to knock the city defenses down, destroyers to watch for subs, a carrier or two if I have flight, and a couple transports full of Marines) to obliterate the enemy's access to the sea. If I have armor on the same continent, that goes after his inland cities at the same time. If it's intercontinental war, I may wait until he has no coastal cities remaining to send in my armor, or not send it at all. As my Marines gain experience, I try to get them March and Commando, along with Combat V ideally (obviously, Lincoln's Navy SEALs are ideal for this). At that point, they can strike deep into enemy territory with lightning speed and brutal effect. I prefer to build them in my HE/WP city, with other cities contributing lower-XP cannon fodder Marines if needed.
I would like to see an upgrade path for them, maybe (off the top of my head) Special Ops, str 30, 1 move, starts with Amphibious (and March?) and has the paradrop ability. Marines and Paratroopers could both upgrade to this unit with Computers or Composites. I'm happy as it is, though.