I've jumped off a bridge twice and it's harder than you'd think to just drown. Typically, we tend to float, which is why people who commit suicide this way use weights. Also, your survival instinct will really kick in, making this way of suicide even more torturous. Drowning is a very painful and terrifying way to die.
This is just my opinion. But I do have some experience with it seeing as how I've jumped from 2 bridges into water. The first time I was able to get to shore since I wasn't too far from it. The second time I was rescued. I was too impulsive to think clearly. I wouldn't recommend it since it's not a reliable way to go.