As some have said above, drowning is more likely the psychological effects of your mind kicking in than physical pain, and with SI activating it's a nightmare during the initial stages. A quick search on Quora and you'll find people saying the same thing: it's not painful at all after a while, but rather relaxing.
Personally, I'm convinced that drowning isn't all that painful because I've had one memorable moment during my childhood. I was about seven or eight, and I was on vacation with my mother and uncle.
One afternoon, we were out on the beach. I was carried by my uncle as he slowly goes forward into the ocean, he initially didn't like the idea of doing so, but eventually obeyed my order after I kept nagging him. Soon later, the beach behind us was quite far away, as I remembered, and we were already facing some strong waves. The clouds came in, and my uncle considered going back in but decided to advance a few feets forward, then suddenly some strong wave crashed into us and I fell off him.
My uncle was okay and still stood up just fine, but I was in the water. I then remember feeling pain from all the water going into me, probably through my nose (or mouth?). Whatever the case was, it happened almost instantly that I barely remember much of it. I do, however, remember that after the water came into my body, I was then underwater for about three or four seconds. I'm confident that it was less than five seconds. That brief moment under the water is still clear in my mind to this day; peaceful, quite relaxing honestly, and the light from the sky above the water shining into my eyes. Those four seconds were over quickly as my uncle pulled me out of the water, and carried my crying ass back to shore and safety.
Anyway, that early drowning accident pretty much convinced me that only the initial stage of water going into you is painful, everything after that is painless. Feel free to correct me.