First off, unless you want areas of the game spoiled right now, LOOK AWAY.
I Repeat, SPOILERS SPOILERS SPOILERS!
For the most efficient path to get EVERYTHIHG, you should probably go in this order. Correct me if I am wrong anybody. Home Waters -> Song Cave -> Krotite Temple -> Open Waters -> Kelp Forest (First South West THEN North West if you know what's good for you) -> Mathalis -> Cathedral -> The Veil -> Sun Temple -> The Abyss -> Frozen Veil -> Sunken City -> The Body.
Hope that helps, and of course, explore every area thoroughly. That's just a rough playbill of the game, you have to actually see the show.