How does the Superfly work on the Apex on regular ground?
On the salt a coil with a smaller footprint will be better. If a coil is too large in the salt it will be more effected by the salinity which is causing the falsing. I had found that using stock 8 1/2 x 11 AT Pro coil vs the Nel Big 17x15 at the beach the large coil worked excellent in the dry, incredible depth but as soon as the salt came into play depth was rapidly diminished and only had a slight depth advantage over the stock coil half the size.
I have a Superfly I use on my Multi Kruzer and find it more stable than the stock coil but use it for open areas with moderate or low trash. I do use it on the beach for towel lines and damp sand but use that combo more after storms up here as the beaches can get riddled with pieces of lobster traps.

I just ordered the Detech Ultimate 9" for my Apex, I haven't used my Apex at the beach yet but a buddy uses his and does very well with the stock Viper coil. 9" might be the limits on that machine for those conditions and I can compare. Got the little larger coil because we have many areas the stock is hitting fringe targets here and just out of reach for the viper making it hard to pinpoint.