I can't mount on any of my mounts at all whilst in any bit of water. If I could, I would have no problems getting the shells in the water.
If I could even use my Fury form as Demonologist it would be easier too, but shapeshifting seems to be disabled on the island.
Next time you try to get some seashells can you please tell me how fast it is for you? The fact that it's gone from 1000 shells to 7000 seems extremely excessive. This is seven times the previous limit.
I play on Smugglers Paradise, so P2P not F2P. It's quieter on this server but I have no interest in playing F2P.
I'll open a ticket about this.