Am I doing something wrong, or shouldn't the waterline necessarily be at the same distance from the string on every form? T

Jay Babina is the OI expert, and he'll probably drop in here soon.

There are lots of ways to line up forms, I think.

I always use the waterline (or a line parallel to the waterline) as the reference for form setup. For me, checking the rocker is something I do after I think I have all the forms 'right', and I don't get too excited about the rocker measurements as long as they are more-or-less correct and the keel line is fair.

I get a (big) handful of straight sticks (strip cutoffs or similar ripped to suit) and fasten/clamp a stick across the waterline on each form. I also attach a stick aligned with the vertical line on each form. (It sounds like a big deal but only takes a few minutes.) Then I 'eyeball' (and use string) to make sure everything is lined up and square. It's easy to see if a form is 'twisted' by using the reference sticks. (The relationship of forms to the strongback doesn't matter much, as long as they are lined up with each other, IMO.)

Once I have everything lined up properly, I staple a few 'temporary' strips to the forms - it is surprising how well just a few strips will 'lock' all the forms in alignment. A final check, and then I remove my 'handful of sticks' from the forms, and get to work.

BTW, string can sag quite a bit, even over a short distance like 18'. I've used fine wire (with good solid end supports) and also thread for alignment.

Taking some extra time with form setup is a good thing!