It sounds like salt water is more effective at this than fresh, so youve got good results waiting for you! Watch out tho, if you go into water that's too deep the pressure will hurt once you're at the bottom.
I found a site with some really useful info and statistics about this stuff, apparently most people who go out this way actually have a rather peaceful, calm experience once you inhale, so I'm feeling much more confident about it.
So far it looks like I'm gonna do it in a reservoir nearby, tie something to me feet and jump and let myself sink. Gonna exhale before jumping so you breathe in water faster, wear heavy clothing just in case and hope I'm sleeping with the fishes in no time. But Im actually kinda looking forward to the last part, where people say they have a relaxing experience. It might actually be worth extending that moment? Idk