When it comes to the death of Regulus Black, the common misconception is that he drowned due to his inability to swim. People think that he felt the water of the lake on his ankles and gave up, realizing that there was nothing he could do. Most people think he went down easily.
That's just not true.
See, Regulus Black was a good swimmer, a great one even. His mother got him and Sirius lessons when they were younger (although Sirius liked it a lot less). Regulus loved the water, any chance he could get he would be swimming. Even once he got to Hogwarts he made it a habit to sneak out of the Slytherin common room, dragging Barty and Evan with him to go swim in the black lake.
So yes, Regulus Black could swim.
That's why, when he walked into the cave that day, he felt calm, collected, confident. He knew this, he knew water. He wasn't afraid when he got on the boat or when he stepped off of it again. He wasn't even afraid when the draught began to bring up his worst memories. He wasn't afraid as he approached the water, looking for something to drink. He wasn't afraid when he touched it.
When the hands started to rise from the water, pulling decaying bodies out of the lake, Regulus couldn't find the energy to panic. As he found himself being dragged backwards, he couldn't even find it in himself to freak out.
"It'll be fine" he thought, he could swim.
And so he kicked up in the way he had done so many times before, a strong swimmers kick, well practiced due to long days spent deep in the black lake at school. He kicked, and he didn't move.
So he kicked again. and again. and again. Over and over and over, until he realized there was nothing he could do.
Months spent on research alone, weeks without sleeping or eating, all of these things caught up with him in that moment.
So he began to panic. He thrashed around hoping to knock a few hands off of him only for them to grab on tighter. He kicked out and punched at the inferi hoping for some escape.
And in his head he screamed for Sirius, for his brother to come save him. He asked for anyone, James, Barty. He begged the universe to send him help.
It didn't.
The lack of air started to get to him as he sunk slowly lower. His head started flashing through memories of lakes and swimming.
He could almost imagine that the arms grabbing him were Sirius, horseplaying with him in the lake behind their vacation home. or Barty floating with him in that black lake. or James hugging him on their first date, (also in the black lake).
As he slowly lost consciousness, Regulus comforted himself in the fake presence of his friends, his family. He stopped trying.
With one last look at the surface Regulus let the memories of his friends bring him down to the floor of the lake.