Honestly I feel like buffing Byzantines on water would be kind of silly. They already are able to challenge all the S tier civs on water as seen in HC4. Koreans and Japanese could get an small buff but honestly rather just nerf the S tier civs slightly like making the Italian discount on fish scale like the Indian cheaper villagers, nerfing castle age Longboats and nerfing Portuguese somehow
They are top 5 or 6 on islands thanks to their cheap ships