A few suggestions that would make Oceania (Middle Age) a more balanced nation...
Give land form stealth to the sea form stealthers. This eliminates the mentioned problem of units getting stuck in provinces and gives them an ability that sets them apart from the other water nations. (Could be done in a mod)
Change the starting force of Oceania to all amphibious units. Under optimal settings, Oceania can be placed in a water province completely surrounded by land. This has happened to me 3 times including in a multiplayer game that I am having difficulties advancing past my starting province because of the mediocrity of the amphibious units and the fact that half my starting units are aquatic. (Once again, no core game code change here).
Oceania is in dire need of some national spells. An amphibious or aquatic summon would be awesome, something along the lines of water/nature that is a little bit better than sea dogs.
Give Sirens 10 leadership.
The above are changes I feel could help Oceania in the early game and well into the game, without unbalancing them in the least bit.
Now as a fan of Oceania, the below changes are biased desires that may or may not be balanced. (Wishful thinking
)
Capricorns to attract amphibious allies in a similar fashion as pangaea attracts Maianeds. Less per turn than their Pangaea counterparts, and just as mediocre.
Sirens to have air 2 in land form instead of air 1.