This is effectively a mini review. I purchased the H3 many years ago and I never seemed to be able to use it correctly. The sound was always corrupted but being such an unusual piece of equipment it was difficult for me to know what to expect, so I was unsure if there as an issue with the equipment or my expectations. I was unable to take any action on this as shortly after I purchased the H3 I moved to Japan for several years. I was extremely lucky that when I contacted the manufacturers several years later they were prepared to help out. This was far more than I expected as it had been years since I purchased the device.
Comparing the H3 with its newer cousin the H2a XLR there are some immediate differences that are worth mentioning. The H3 only had a mini jack output and so the quality of its output was not quite as good. Also it required an internal battery for power where the H2a now uses phantom power. I did like the more direct diaphragm access on the H3, but I suspect the metal shell of the H2a is much stringer and will increase its longevity.
The biggest feature of any of the Aquarian products is they are excellent value for money. At only a few hundred dollars they become a piece of equipment that every sound designer should have, and compared with other commercial hydrophones they can be as little as 10% of the cost of other underwater mics.