One of the things I love about this game is it involves a level of skill, ok not a lot, but at least some level of thought and judgement. Any dumbing down of the aiming mechanism would be a bad thing in my view, and I wouldn't say I'm much good at it yet either. It's a 10 minutes to learn, but a lifetime to master kinda thing, and that's good.
Also I'd see no point in 'fixing the red line in the vertical' as between one broadside and the next so much will/could have changed in terms of range, and then there's the roll of your ship as well. If we were sailing on a flat sea, with both ships on parallel courses, at constant speeds then 'fixing in the vertical' would work, but thanks devs, we don't have that. I'm learning to use a cloud in the sky, the enemy name tag or a yard arm to give me an idea of the range on this shot then adjusting/guessing the next shot appropriately given whats happened during reload time.
If you still think something like this might work, take a strip of 'Glad Wrap' the height of your monitor, put some marks/graduations on it and lay it on your screen it might help you remember your vertical elevation but I don't think it'll help you get more hits, too much else is changing. Getting hits at short range is easy, getting hits at longer ranges isn't, that's historically true, accept there's a skill to learn here and enjoy that.