Despite Misty Terrain, I think Scald is still the goto option for STAB on this Pokémon. Remember that Terrains don't affect airbone Pokémon, and thus a prominent number of OU relevant mons face the risk of being burned if they take a Scald(i.e. Skarmory, Rotom-W, Tornadus, Thundurus, Landorus, Zapdos, Gyarados, Mega Pinsir, Pelipper, Celesteela), as well as her three fellow Guardians, which all dislike a burn, especially Tapu Bulu. And of course, Scald can always be useful if Tapu Fini for some reasons stays out until after the Terrain wears off. Surf doesn't have any useful side effect on its own anyway, and I don't think the extra power allows Tapu Fini to get any important extra OHKO/2HKO