While Walani's pockets are full, if she is standing on land and holding her Surfboard in her hands next to a body of water, she can drop it with the Right Mouse Button and it will be lost to the depths. This will not occur if she is using a boat; it will only happen if she drops it from land into water.
I think what's happening is the Surboard has two states for being on the ground, one for land and one for water, and the one for land doesn't have whatever tag makes items float in water. Therefore when the game attempts to drop the Surfboard as a land item, it hits the water and immediately sinks.
Edit: I realized that this will also affect crafting Surfboards with a full inventory and hands full.
Steps to Reproduce
1. Pick Walani 2. Give yourself 15 extra surfboard_items 3. Stand near the island's edge 4. Use the Right Mouse Button to drop the Surfboard
This issue has not been confirmed by a developer yet.