As Masis stated above clams unfortunately can't be stored in any helicopter on the server. This didn't used to be the case, the reason for the change was because pilots were hovering 1m above the water and their friends would get out and go harvest the clams. I believe the development/staff/management team didn't like the fact that players were doing this and therefore removed it!
As above you can't use a heli for them however if you have Apex i would suggest using the RHB Boat as you can make 200-300k per trip and takes about 10-20 minutes.