I agree drowning is probably a hard way to go, but with weights and in deep enough water, where it would be difficult for someone else to get you out quickly, your chance of success is good. Forget the bathtub. The SI would make it difficult to keep yourself under water. If you're absolutely intent on CTB, by attaching weights to yourself, you eliminate the ability of yourself to be able to get out of the water, as long as the weights are heavy enough. The only SI you'll have to overcome is jumping out of the boat into the water knowing that you'll sink just about instantly. I don't know how strong you are, but having enough weight attached to your person is key, since you may be able to overcome a small amount of weight, at least for a short while. Good luck if this is what you want to do.