Basically, the idea of
A Legendary Ocean is as follows:
All water monsters get +200 ATK/DEf, simple enough.
Either player can normal summon or set level 5 water monsters without tribute.
Either player can normal summon or set level 7 water monsters with only one tribute (Levia-dragon loves this).
Water monsters usually level 4 can now dodge the likes of
Gravity Bind and
Level Limit - Area B.
Any card effect that requires "Umi" in some way, may use
A Legendary Ocean instead.
Since
A Legendary Ocean is treated as Umi at all times, you may only have a total of three of any combination of Umi and
A Legendary Ocean (so, 2 of one and 1 of the other is fine, but not three of both).