Drowning and poisoning can be good methods. But you should understand that the media/movies is not accurate. They make it seem like these are easy, quick, clean, and peaceful methods, but the reality is very different. I'm not trying to dissuade you at all but more so give you some information.
First, drowning. I've personally had a couple of close calls while surfing in big storms over the years. It is painful. It's not peaceful. And it's definitely not as quick as you'd think (maybe it is but it feels like an eternity). I'm not going to go into all the details but I can assure you, it is not pleasant. Also important to add is that if you are a good swimmer, good luck. Your si will kick in and it will make is extremely difficult to succeed. Tying weights is something that people recommend, and while it will work pretty effectively, I personally couldn't do it. But some people have no problem doing that so it really varies from person to person.
Second is poisoning. Depending on what substance you are using, the results can be very mixed. A lot if not most poisons can be very painful and slow (and not effective). Make sure to choose your poison carefully if going this method. Researching (what to use, how much, etc.) is important. SN is popular but yes sources can be hard to come by. Just don't drink bleach, or weed killer, or rat poison.
Best of luck, bb