One Fish, Two Fish, Half Fish, Marine Biologist.

Notes.

"You're already here?" Izuku asked as he walked up the few steps to the platform

"Yep. Waitin' for you." Katsuki looked over his scarred shoulder

Izuku blushed slightly and sat down next to the blonde with a notebook.

Katsuki seemed to notice and frowned confused.
"What's with the book?" He asked as Izuku settled

"Well I have something to ask." The other took in a deep breath

Katsuki rose an eyebrow.
"Don't you always?"

Izuku nodded with a small chuckle.
"Yes, but this is different." He said

Katsuki waited for an explanation, but it never came since Izuku was staring at his book.
"Wanna elaborate, Nerd?" He asked gesturing towards the notebook

Izuku flinched, startled.
"Oh right." He laughed awkwardly

"I was wondering if I could take notes on you. You see there's not a lot of information on Merfolk so studying you and asking personal questions would be really helpful to the research." The marine biologist explained

"Though, I understand you like your privacy. So the notes will only be for me unless I ask you otherwise or if you don't want me to take any notes at all then I won't." He babbled

Katsuki frowned and looked down at the water as he processed the information.
"How am I supposed to trust your word?" He asked

Izuku then smiled pulling a rolled up bag out of his pocket.
"That's what this is for!" He said holding up a clear heavy duty item

"It's a waterproof bag." The other said when he noticed Katsuki looked confused

Izuku starting putting the notebook in the bag.
"After I'm done I'll put the note book in here then give it to you and you keep it so you know for sure that I'm keeping my word." He explained as he dipped the bag in the water

After fully submerging the bag he lifted it up and opened it to pull out the notebook.
"See? Perfectly fine!" He handed the book to Katsuki

The blonde looked at the notebook seeing if it was wet. He realized it was completely dry.
"I guess that's okay then." Katsuki shrugged

"Really?" Izuku asked with wide eyes

Katsuki looked over at the other and nodded.
"Yeah, it's fine." He said handing over the book

Izuku smiled again as he took the notebook.
"Awesome! So would you mind answering a few questions. You can say pass on any questions you don't want to answer." He said pulling a pencil out of his cargo shorts

"Okay." Katsuki nodded again understandingly

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"What's with the getup?" Katsuki frowned up at the other from the water when Izuku approached him

The marine biologist looked down at his wet suit and gear. He trained for two weeks so he would be able to do this. Katsuki didn't know though.
"Well Kacchan doesn't have to stay above water to swim so I thought it would be more fun if I swam fully submerged in the water with you." He answered

"What's that stuff?" Katsuki asked

Izuku looked over his shoulder.
"This is an oxygen tank and this is underwater scooter. Since I'm human and the other stuff is pretty heavy I can't swim as fast as you so this will help me get around more quickly." He explained as he pointed to each item

Katsuki frowned.
"It seems complicated for humans to swim like I do."

Izuku nodded.
"Yeah because we only have lungs and we don't have tails. We just aren't made for living in the water." He shrugged