Just my 2 cents, for whatever its worth. When I was 15 I (accidentally) almost drowned while snorkeling when I underestimated how long it would take me to get back to the surface and/or overestimated my ability to hold my breath. It was honestly the scariest experience of my life. So that terror people talk about is real, or at least it was for me that time. HOWEVER, I didn't want to die. Surely that makes a difference? Maybe, beyond overcoming SI as required with any method, it isn't bad.
In fact, in the time I've been contemplating CTB, I've considered drowning as a possibility for myself. I haven't ruled it out, although I'm currently leaning toward something else. I relate to the reasons that you described for why drowning appeals to you though. For me, additionally I sometimes feel like maybe that was supposed to be my time, and to drown now would on some spiritual level be fixing the universe's mistake.
As to specifics, for me with any method the #1 priority is eliminating as many failure scenarios as possible. When I'm in that moment, ready to go, making that final decision that I'm ready and then literally taking the plunge, the peace of mind that the decision is absolutely final would bring me comfort and I believe courage as well. I don't want instinct messing it up for me, which basically means restraints or weights. I really don't want to bind my hands or feet because I feel like that for sure would induce an unnecessarily immediate and severe panic response. What I'm currently thinking is taking 50+ pounds of steel chain (buy from any hardware store) and wrapping it completely around my torso making sure to go over my shoulders, under my arms, in between my legs, etc. Basically wrap it so it isn't removable, using padlocks to secure it. I figure this would induce less panic than binding my arms and/or legs while the weight ensures I immediately sink and can't get back to the surface. Plus, being wrapped this way would anchor me sufficiently that my body won't wash ashore anytime soon. Maybe a few body parts will come loose over time due to decomposition or feeding, but by that point they might not even be recognizable as me even if they do come ashore. Again, if and when I do CTB it most likely won't be by drowning, but if it is, this is my current plan.
A weighted backpack could certainly do the job, but I would worry about making sure it couldn't slip off or be removed if SI overwhelms. Getting it secure enough to be sufficiently certain just seems like it would be needlessly complex and still risks underestimating SI. My method uses the weights themselves to secure the weight to my body in a way that it isn't physically removable without the key or bolt cutters, neither of which would go in with me. I live (and die?) by the KISS principle- Keep It Simple Stu… (won't finish the word because I'm not sure if the forum has any word filters and don't want my post deleted by a bot)
I'm not intentionally critical. My method may be completely wrong for you. Just hoping to add to the pool (lol) of ideas, so you can consider as many things as possible and decide what's most appropriate for your desires and resources. Whatever you decide to do, best of luck and I hope you find peace.