You can't just walk up to it, you have to walk up the poles (they form a ramp) on the left side of the boat. Then you can access what's inside the case.
The weapons in your inventory shouldn't affect whether you can open the case, however if you already have one of these items in your inventory you won't be able to pick it up... Since you haven't opened the case yet, I'd imagine that it's safe to assume that you don't have one.
Make sure you've got an empty slot, walk up the ramp to the left and open the case.