The biggest one is that Level 5WATER monsters in your hand are now Level 4, so they can be Normal Summoned or Set without Tribute! WATER monsters that were Level 7 are now Level 6 and can be Tribute Summoned with only 1 Tribute, instead of 2. "A Legendary Ocean" is also treated as if its card name were "Umi".
This means that other monsters that gain a benefit from "Umi" will gain their benefits from "A Legendary Ocean" as well. (This also means you can only have 3 copies of "A Legendary Ocean" or "Umi" in your Deck, you cannot have 3 copies of each).