Bella's legs thrashed around in the black waters of the lake, desperately trying to kick her up, up to the bright light where she knew the surface to be. Her arms were already hurting, pressed tightly to her sides by a simple binding spell. Something touched the bare skin of her foot and she jumped, or would have, if she hadn't been underwater. Weirdly enough, that brought a little sense back to Bellatrix, and she stopped her frantic struggling to think. She needed to somehow break the binding spell, but she didn't have her wand, they had taken it away from her after she had been brought to a cell to await her sentence. Of course, they hadn't believed her claims of innocence, of having been under the imperious, and that had led her here, floating, in the depths of the Hogwarts Lake, with no wand. She had been without wand before, like the time Potter had taken hers and her master hadn't let her obtain a new one for a week „as punishment" (along with the cruciatus curse of course). What had she done then? She had still used magic. She could do wandless magic. Yes! She could do wandless magic!
Bella tried to focus, to break the binding curse, but she was running out of air fast and concentrating was harder than expected. The women closed her eyes, which she had been straining to hold open until now, and willed all her energy towards the spell, loosening it slowly until finally, she was free. Only she wasn't, because she was still drowning, and even if she managed to get to the surface before she ran out of oxygen, which was unlikely, they would just force her back in, with stronger bindings, or find some other method to enforce the death penalty, one that wouldn't fail.
Bellatrix needed to find her wand, she knew they had thrown it into the lake with her. Gathering the last ounce of strength she dove downward into the depths of black water, the light above her becoming more distant with every second. When she had nearly reached the ground, or at least she thought she must have, she couldn't see a thing in the darkness, her hands touched something slimy. She quickly paddled back a few meters, before realizing that must be the seaweed covering the whole of the lakes ground. She knew there were Grindilohs in there, so she probably wouldn't be able to swim through it. God damn it, she wasn't going to die this way!
She could try using more wandless magic to blast a path through the huge plants, but that took a lot out of her and in her weakened state she didn't think she could manage anything strong enough without falling unconscious. This was ridiculous, she was innocent! They could have so easily tested her, two drops of verita serum would have been enough, but no: „Something as valuable as the serum of truth ought not to be wasted on someone this obviously guilty." And she had received the death penalty. The death penalty! She hadn't even known that was still legal in magical Britain.
Her lungs started burning as she remembered her current situation. Bella probably wasn't going to be able to stay conscious much longer, so she decided to just risk it. She dived straight downward through the weeds scratching at her arms and face, and she thought she could feel a hand trying to grip her leg, but it slipped off again a moment later. Then, her hand collided with the ground, sinking into the mud and stirring up a big cloud of dirt around her. The woman had no time to wait for the dirt to settle, so she started blindly digging through the sand turning over rocks, her fingers getting caught on bits of leaves. A bit of seaweed caught in her hair and as she tried to untangle it she realized what she was holding. Gillyweed! She was holding Gillyweed! Without a second thought she stuffed it into her mouth, and finally, finally she was able to breathe again.