I don't have footage of it, so here are the instructions:

You need to sink into the water with inventory drop. After you start sinking, you need to cast a spell. I like using D-Dark. Right after you do the spell, tap left or right to cancel out of it. Since you didn't let the Knight to sink very far, you can jump out of the water holding your jump button. The Knight will have some shining ball in his face and he will start ascending.