This was on my absolutely must do list for our trip to Vanuatu.
Unfortunately Cyclone Jasmine had damaged the underwater post box itself and the roof to the post office a couple of days before we had arrived. We were told about this later in the day.
I bought my post card (400 Vatu) and took it for a swim, only to discover that the post box itself had a broken door. I tried a few times to wedge it in there but my postcard wouldn't stay and I had to swim after the card several times. I came to the conclusion that I had technically posted it so I posted the post card in town into a normal mail box. There were many families with disappointed children who weren't able to post their postcards.
I would visit the post office again. The whole concept is great and I still really enjoyed the novelty of snorkelling to and around the post office. I hope it is back up and running soon.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC