Iron Gator is certainly winnable by MECSF. With a bunch of non-team players on both sides, either side can win. If the Seals are working as a team or at least have a few guys guarding the docking bay and MEC isn't coordinating at all, the MEC is going to have a hard time.
But if MEC has at least a few clued in players, then they can get on board and capture a flag and have a chance from there, even if the Seals are working as a team. Here are a couple ways to do it:
1) SPEED: This is probably the best way. Spawn at the shore, sprint to a jetski, hop on and head for the docking bay. You can get there faster than Seals spawning at all but one of the onboard flags. IIRC, there are at least four jetskis. If all four are squad leaders, then any who make it onboard can be used as spawn points even without capturing a flag. And the engine room flag is right by the docking bay.
2) SKY: The transport chopper can fly high towards the back of the ship (so it is safe from the ship's gatling gun) and passengers (whether along for the whole ride or spawning from SLs on board) can jump and open their chutes right before hitting the deck. Most will land near the flight deck flag, in which case use the abundant cover near the flag. You may also be able to land near one of the doors on the rear portion of the superstructure. If so, head inside rather than try to cap the viewing flag. May also be able to land on the elevator and get into the hanger.
3) SLOW SEA: The inflatable boats have a machine gun, but they also have additional passenger spots that can be even more useful. A support guy can shoot tear gas into the docking bay, which will provide smoke cover for teammates climbing the ladders, as well as possibly catching a few Seals before they get masks on. Two or three inflatable with 2-3 guys each should be able to fight their way onboard.
4) REALLY SLOW SEA: The MEC APC is amphibious and provides awesome covering fire into the docking bay. If coordinated with a boat or two it shouldn't be too hard to get out of the water. But whatever you do, don't leave the APC behind in the docking bay. If someone does and a Seal gets inside, you might as well forget about anyone else getting in that way.
There are a few other things that make any of the above easier:
1) A decent Hind crew. Should at least keep the Apache busy. Should use the TV missile to take out the gatling gun and missile launchers. If they do that AND take out the Apache, MEC will have NO trouble getting onboard.
2) Smart squad switching. Anyone who gets on board needs to become a squad leader if they aren't already. Other than at the very start of the round, anyone spawning should check for an onboard squadleader and spawn from them if there are no flags under control.
3) If in an inflatable, blow up the explosive barrels in the docking bay before climbing the ladders. Otherwise a Seal will do it for you once you are next to one.
4) Shore AA. Can at least use lock-on to keep the Seal chopper worried about missiles. Be careful not to fire when the friendly choppers are near the ship though. As we all know, AA missiles prefer friendlies.
Hopefully this post will help at least a couple MEC teams make a game of it. But yes, this map was obviously playtested before grappling hooks were removed from the kits. I'd love to hear why they were removed in the 1.12 patch.