I think naval vs. aerial is the main problem for this map. To put it bluntly naval units are far too slow. Especially when cover is available in the form of the icebergs, it allows the helicopters to run circles around the ships. Add in the helipad and service depot smack in the middle of the map for easy refills/repairs and it really doesn't make any sense to buy a ship if your team has control of the middle.
How would I fix this? Make destroyers and missile subs very, very effective against aircraft. Helicopters already have such great mobility over ships that they should still be able to avoid them relatively easily and strike land-based targets. Increasing the durability of naval vessels against air pushes more people to get naval units, which to me (Raap can disagree) was always the intention of this map.
I realize this may affect the balance of other maps as well, so if there is concern this will make Destroyers and Missile Subs too powerful I would say nerf them against land targets.
And just one final thought, I'd also get rid of the turrets and flame towers guarding the naval yard/subpen. Again, I don't think you need more reasons to discourage the use of naval units