Ok let's imagine Will and Hannibal moved in a Mediterranean place, in a house on the beach. Obviously Will loves to be in the water and go snorkeling and admire the pretty fish.
Hannibal is not really a fan. He loves swimming but in pools where he can see what lives beneath him. As a predator himself, he needs to be constantly aware of his surroundings and asses the potential harm. Will tells him every night about the many types of fish he has seen that day.
One day however Will convinces him to come into the water. He promises him there is nothing to be afraid of, there are only harmless fish. Hannibal gives in.
The water is nice. And clear enough. And Will promises to report every weird thing he sees beneath them. They don't go where they can't touch the bottom.
"I have to admit, Will, it's not that bad."
Will doesn't reply but takes off his snorkeling glasses.
"What else is there?" Hannibal asks seeing the mixed feelings on Will's face.
"Nothing. A couple of fish."
"Will. What did you see."
Hannibal is frozen instantly. Him, the apex predator, is facing a potential death threat and he doesn't even know what it is.
"You promised." He whispers as a small grin appears on Will's face.
"I promised nothing will harm you underwater. I never made any promises about myself though."
Before Hannibal can react, Will has already "attacked" him, pulling him underneath. Now that his attacker has made himself known, Hannibal has no issue with fighting back, especially because it's the idiot that he had fallen for.
At least their little fight had for sure scared any underwater predator.
By the end of the day they are both cuddling in their bed, Will's head resting on Hannibal's chest.
"By the way, there was a moray eel right next to your left foot. I attacked you to scare it away. Did you know they have really bad eyesight and-"
"Are you telling me this just now?"
"You would have freaked out and where would that have led? You are okay, aren't you?"
"I could have been dead by now."
"No way. Do you think I would have let that happen on my watch? I would have killed it with my bare hands and then I would have cooked it for you."
Hannibal is pleased with Will's answer. However he still prefers to stay on the shore.