Hi Oosh,
We have a friendly local council worker who drives the tractor that towes the beach sweeper (surf rake). He starts at 4.30am most mornings and sweeps the beaches to remove weed and debris.
We swap him beer for boards, bars and lines, kites

Whatever he finds. He even found a pneumatic speargun recently.

Check with the rangers or council to see who cleans the local beaches and try and track down the guy who is responsible. If he's a good guy you may be able to get gear back. We have established a good relationship with him over the years.
Sorry I couldn't be more helpful.
I wish you good luck. Write your name and phone number and "Reward" on your board. It will come back to you 90% of the time.