Now that that was solved, what was she going to do next? She couldn't go up again, as there might be someone there still, to make sure she was actually gone, and she could only stay underwater for so long before the Gillyweed wore off. First of all she should probably get out of this seaweed again, before she was attacked by some creature. She was pretty sure she could defend herself now that she had her wand back, but she was still quite weak and slightly dizzy from the lack of oxygen before.
With that, Bella swam up again, through the slimy plants and into the clear water. Had she been able to, she would have breathed a sigh of relief, this way she just allowed herself a weak smile. Now what...? She could just go up to the surface and hope they had been so sure of themselves that they hadn't put up any detection charms, but that was risky. Maybe she could find a cave or something to hide out in until she was sure the coast was clear? Yeah, that was a good idea.
She made her way slowly over to the part of the lake that bordered the mountains, all the way swimming underwater, though closer to the surface then before. It was a long, tedious swim, but finally she reached the rocky border of the lake. This took another considerable amount of time, in which she discovered that there were not, in fact, any caves that led from beneath the lakes surface to above the water, which meant, no place for her to hide out for a while without drowning. Grasping at the last bit of hope she had left, Bella made her way further around the lake.
Luckily her dizziness had worn off a little, and the Gillyweed improved her ability to swim fast quite a bit. She looked for any further caves, but, of course, with no luck, until she finally made her way to the part of Hogwarts that directly bordered the lake. As she swam along the steep wall she noticed a pipe of some sort, probably a drain, leading out into the water from inside the castle. This was her salvation! The pipe was blocked by thin metal bars that she easily blasted away with a swish of her wand. She had been the dark lord's most powerful servant, after all, although not willingly so. She cast a low intensity lumos and swam forward into the darkness of the Hogwarts pipe system. Soon she found a pipe leading upwards and out of the water. Drying herself off she clambered into the new tunnel. This way she slowly, carefully made her way up into the school, until faced with a rather troubling discovery.
The closer to the actual castle she got, the more tubes branched of the main ones, and the smaller the pipes became. This meant, she couldn't find a way out going up, and she would have to stay in the tunnels or find a way out outside the castle again, which was unlikely to happen. At that moment though, Bellatrix was not worrying about that anymore, she was just tired and exhausted. Exhausted enough that she only barely managed to perform a few weak warding spells before her eyes closed and she fell asleep.