As far as I know as little chance of failure as possible is going to be harder to get with the extreme methods like jumping off a bridge. Apparently 4% of people who jump off the Golden Gate bridge survive.
Also jumping into water doesn't always give an immediate death, and you could just die from drowning instead.
I would also consider how your body will be affected if you do survive. Shotgun has a really high chance to kill you but if you don't aim well you can just blow your face off.
If you really want something adventurous over something safe if you survive I suppose you could look for a higher place to jump.
People have survived landing face first from sky diving (I think this person had a failed chute that deployed but still), and someone has fallen 200 feet onto a flat rock, then another 100 feet onto another flat rock and survived. These are extreme outliers, but I would personally never choose to jump.
I personally think sodium nitrite is the best method. As far as I know it's one of the, if not the most , safe methods to use if you survive. If you do it right it seems to have a very high chance to work. It's got some physical symptoms like nausea and a head ache (I'll give a link which mentions it all). It's not anywhere near cinematic though.
https://sanctioned-suicide.net/threads/stans-guide-to-sn.27535/