Damn. The 2' deep tub sounded feasible but reading through the comments, I can see how difficult it would be unless you were heavily sedated/unconscious. Even then…well, as OP says, it didn't end up working
I remember an episode of Forensic Files where a married couple were staying at a hotel and the husband killed the wife in the bathtub, staged it to look like she had tried to grab the towel ring and the ring dismounted, causing her to fall into the tub and essentially drown.
Investigators were curious how a petite woman would've have had the strength to rip the towel ring from the wall and they tested towel rings in other hotel rooms using much more weight. All in all, the reality was the husband had held her down under the water and killed her. Motive: Insurance money. He was going to try and sue the hotel, etc.