Personally I wouldn't consider it a legendary. However;
Phione's mythical and/or legendary status has been heavily disputed due to the fact it is easy to obtain lots of Phione on the same game card without cheating should one have a Manaphy or a Phione and a Ditto, even official sources have differed on the matter.
However like I said, from my point of view it isn't a legendary. While it may being the offspring of a legendary and part of the Sea Guardians, it's stats are subpart for a legendary, and I really don't see what it represent's that Manaphy doesn't already. Additionally it's too easy to get to really amount to being a legendary which are generally considered rare.