Legendary Ocean probably better you can summon Giga Gagagigo who is 5 star with legendary ocean without a tribute nothing like 2650 attack beater first turn when you end up with legendary ocean and him in your hand. Also levia becomes 1 tribute too.
Not sure about Umiiruka being strictly better for Shark decks though. Legendary Ocean allows to summon 4 star monsters, such as, for example, Gagagigo using the effect of Hammer Shark. Even better, if you have 2 Hammer Sharks in your hand plus a random lvl 4 or lower Water monster, you can summon 3 monsters in 1 turn if you have Legendary Ocean on your side of the field.