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.