It doesn't come on instantly, and isn't that strong a painkiller until long after you'd be dead by natural drowning. You'll be struggling desperately to stay above water as you fight against sleepiness, and will probably drown before you are unconscious enough to not feel the panic and pain. Alcohol even has NMDA antagonist activity and is a more effective painkiller at sub-coma doses, but both options have the same drawbacks - too slow, not injectable. Even N probably wouldn't hit you quickly enough to not have at least a few minutes of experiencing your drowning, and at that point in its onset, it wouldn't be much of a painkiller either.
If your goal is to drown without feeling it, then your only real option is to use something injectable, because that is the only almost instant way of taking drugs. I don't think 1,4BD is suitable for injection, and it requires a dose so large that drinking it is the only viable way. 1000mg of ketamine though, should eliminate the pain factor, and could be combined with heroin for extra pain relief/sleep, and any benzo, like 100mg diazepam, to keep you calm and asleep, and all of that would fit in a small syringe. That's the only way I can imagine a "sea death" being anything less than horrific.