I'm sorry you find yourself in that position.
The method can be effective, but I have to be truthful that it is far from a pleasant one. Survival Instinct kicks in fast, your brain will make you change your mind quick and you will get very anxious trying to survive. It may take some time too, sometimes up to 15 min, but that can vary. Also the drowning experience itself is described as quite painful. The lungs get filled with water and it feels pretty much like fire. At least in a river there is not salt in it, which can make it even worse. But still, not great at all. Also, some people survive it, they wash ashore or just make it due to the si. I've read that about a 60% rate of death.
This is not to fear monger or scare you, but I think you should have the info. Others can provide more I'm sure.
Also, if you have a sort window and family nearby aware of your tendencies, might not be the best idea. but that is just my opinion.
Whatever you end up doing I hope you can find some peace. Also, if you end up not doing it (which would be ok) you can come here and vent whatever whenever and we will be around for support. Big hug <3