It reduces the Level of monsters in hand which makes Level 5 monsters to be normal summoned without a tribute.
It gives 200 attack and defense to monsters.
Card effects that has a synergy with Umi also benefits from this card since this card name is treated as Umi.
Only WATER monsters can benefit from its effects.
Unlike Umi, it doesn't decrease the attack and defense of Machine and Pyro-Type monsters.
Your opponent is also affected by its effects which could be bad in a mirror match.
Lowering Hammer Shark's level means that you can only activate its ability twice.
Leevl 5/7 Water monsters
While A Legendary Ocean is face-up on the field, higher level Water monsters will be easier to summon. Level 5 monsters like Orca Mega-Fortress of Darkness can be normal summoned without tribute. They already have an attack advantage compared to most level 4 or lower monsters, and their attack is also increased from Legendary Ocean's effect. Levia-Dragon - Daedalus can be tribute summoned using only 1 tribute, making it much easier to use.
You can either bring more Legendary Oceans in the deck, or use card searches like Planet Pathfinder, or An Owl of Luck to search from your deck, ensuring that you get the field spell, which you can potentially get very early in the duel.
The reducing of water monsters level by one might be considered somewhat op but the 200 boost isn't that great, besides if "this shit" is really such a nuisance then why not just put in s/t destruction/negation? Surely it's not that hard...
You realize that Legendary Ocean is a card, not a skill, right? The card is bad and is a dead draw in most cases. If you want a Water Deck you're better off just running a pure Hammer Shark deck rather than trying to brick yourself with Bugroth and Legendary Ocean
Real quick, has anyone gotten this card from the card trader? Or even seen it in the card trader? I would like to get 3 of the card just for fun (i mostly run water decks) but haven't found this or Uminotauros in the card trader yet. 3 copies of Ocean Dragon Lord thought...