Boat Variant to Balance the Game and Make it More Fun!
Hard to say. I haven't actually seen fleets as a problem. A large enemy fleet can cause a lot of headache, but a large well positioned fleet will quickly attract a raider. Having one of your large supply slots continually raided is pretty frustrating, which means you will need another large fleet to protect the first large fleet, thus cosing you two large supply slots. Not to mention at least half your available ships. But you are perfectly right that a player relying solely on land armies will find it difficult to face amphibious assaults and I also agree that some players have a better opportunity to take advantage of large fleets.
If you feel like experimenting, I'd go for your option 1. The third option is too draconian by far and will make Greyjoy's and Baratheon's positions untenable. The second option seems harsh as well and almost designed to crush an early Baratheon hold on King's landing.
Tell us how it goes!