To Midoriya, the sea is life-giving and benevolent, but it is ultimately not his home. He doesn't go out on boats often. He holds no special affinity for the water, though the beach is bittersweet to him for other reasons. He knew early on he wouldn't specialize in water rescue as a Hero. But he can move about easily with his powers, having the super strength to swim through a squall and the ability to float to prevent himself from drowning.

He is of course committed to not harming the possessed too much. He's in training to be a Hero, a combination public servant and emergency worker. Capture is his aim, but as the battle chaos ramps up to an overwhelming degree, he is more focused on the Leviathan proper.

So, even though Danger Sense alerts him at the last second of an incoming attack of an indeterminate sort, he is fielding pings from many directions and cannot react to them all at once. He is taken by surprise as his ankle is grabbed by what looks like a sea monster--

Gughgh! He chokes underwater and opens stinging eyes to find his assailant in the blue-green tide. Is this person part of the Leviathan's cadre or a Sleeper? Unlike Luca, Midoriya doesn't want to use excessive force, especially on someone who looks so young. With his free leg, he aims a kick with his armored shoe to the empty space next to Luca's head, hoping the sudden burst of water pressure from his super strength either dislodges or stuns him.