Dropping items is much trickier than it seems in Subnautica. By default, hover the mouse cursor over the item you wish to be drop and press the RMB.
However, in this game, you can’t drop items from your inventory if you’re in the Cyclops, Escape Pod, or Seabase. If for some reason you still find yourself having trouble dropping your load, exit the game entirely and log back in. And if that still doesn’t work, your best bet is to send the developer a detailed email of where you are, what you’re attempting to do, and every other circumstance that may be useful.
Subnautica is still in early access and as such, still in active development.