Keep in mind the following
- you dont necessarilly have to have your foot over the fins to do a long arc turn, but you do need to have your weight over your back foot to some extent
- just leaning to turn, like you might do on a skateboard, only works for subtle turns or if you have a lot of speed. for most beach waves after the first bottom turn, its not going to work. You need not only weighting but strong upper body movement - twisting your whole upper body toward where you want to go (look then twist).
You cant just lean or do subtle things, you have to make an effort to turn, esp on a longer board. Early on someone told me the best way (even if it feels weird) is to actually point with your index finger where you want to go with a loose upper body, so your shoulder follows your hand and your lower body then 'untwists'
- foot over the fins works for much sharper moves, like shortboard 180 cutbacks and so forth. You can do the same on a longer board, but you have to actually move your feet back. Walking back and forth on a longboard (9ft) is a given when you want to do sharper turns; on a 7ft6 you probably have to do the same but as a much more subtle movement. So it move back, turn, move forward. How you know where to move your back foot to is a bit of a lesson to learn; but you can figure out where you foot is when you pop up and then figure out how far it needs to move back - on a 7ft6 it may only be 6 inches or so. Then you just move that distance and train your body how far that is. Trial and error really - which can be hard if you are riding short waves, but you can do some of the trial on the white water as you have already noticed.
Surfing is a matter of weighting and unweighting, but on a longer board actual movement back and forth is necessary for the sharper turns or if you dont have much speed.
You should be able to do pumping turns (weaving up and down the waves) without moving your feet, but you will still need to do the weighting thing and the upper body thing.
This well described at this link, which is a blog from waikikichan who also posts here - see both the weighting issue and the upper body issue, clearly explained through some great photos.
http://alohaki.jugem.jp/?day=20140514Also check out videos of good longboarders - Alex Knost for example, and just watch his technique and how his weight moves and his feet move and his upper body moves.
Good luck -