If I read right, you mean there was slack during the retrieve. Under most conditions this is a function of pointing the rod nearly into the water and following the fly with the drift, but in big surf there's a whole 'nuther element to it. If you have a floating running line, you'll have a problem. I've been fine with intermediate running lines (SW QD, Rio Deep Sea, etc).
I used to use a full sink; it's easier to aerialize long back casts and feels more like a normal line (on speed). This is nice if you'd rather work out the kinks as you feed line into a long line length as you cast. I do recall it being better at cutting into waves, but I often try to manage the line around the breakers nowadays with intermediate running lines.
I think if you use an intermediate running line you'll be OK and can keep the dense head (if you like that). The full sink will give you a more standard line feel and the approach will be to cut deep into the shore breaks as you retrieve. When the breaks are turbulent enough, I would guess this could actually work against you(?).
In any case, float running line is great for mending in rip currents but gets sloshed by surf.