I will leave the discussion of the relative significance of the Emperor's research and discoveries to actual marine biologists.

But to answer the first part of the question, yes, Hirohito did publish his research both during his life, and some was published posthumously. In terms of recognition for his contributions to science, Hirohito granted an elected Fellowship of the Royal Society in 1971.

Much of his research was centered around observations of different species of Hydrozoa published under the aegis of the Laboratory on Imperial grounds in which Hirohito had researched biology since he was a young man in the company of the lab's director and biologist Dr. Hirotaro Hattori.

Much of his life's research including the samples he collected and analyzed as well as his original observations now comprise the Showa Memorial Institute (Showa being Hirohito's posthumous name) as part of Japan's National Museum of Nature and Science.

My primary source for this was the biographical memoir/obituary for Hirohito published in the memoirs of the Royal Society written by noted botanist Edred John Henry Corner which I would recommend reading for a far more in-depth account of Hirohito's life in science as told by a peer from his field.

Corner EJH (1990), His Majesty Emperor Hirohito of Japan, K. G., 29 April 1901 - 7 January 1989, Biographical Memoirs of Fellows of the Royal Society, Vol. 36 (Dec., 1990), pp. 242-272 (31 pages) https://www.jstor.org/stable/770088

Aquatic biologist here (I do both marine and freshwater work). I never interacted directly with the Emperor Showa, but have one degree of academic separation from him, so I can make a few comments. Maybe someone who actually collaborated with him can weigh in later, although I think most of them have retired and may not be active on Reddit.

He was a legitimate biologist, and did taxonomic work both on slime molds (which are terrestrial) and hydrozoans (think, jellyfish), and also published several florulas (accounts of the plants present in a particular region) for imperial properties. His interest was genuine, so this was more than just an idle pass-time for him; I think it is legitimate to say that he would have been a professional naturalist and systematist had he not had the inconvenient circumstances of being a god-emperor by birth.

Now, the real question from OP was whether or not he made scientifically important discoveries. I want to give a little pushback on the premise of the question, because it can be very difficult to assess any individual’s impact. We have a kind of a cult of the lone genius scientist, and while there are certainly people who make outsized contributions, there is also a lot of journeyman science which isn’t super exciting in and of itself but is incredibly valuable long-term and should not be dismissed as idle entertainment or ‘stamp collecting’. A lot of taxonomic work is in this category. So, there has certainly never been his equal in the taxonomy of slime molds of the Imperial Palace of Japan. Is this your priority for discovery? Maybe not, but as we experience a climate change/mass extinction event of a scale that has been matched only a handful of times in the history of the planet, having baseline, detailed floristic information on particular locations makes it possible to assess the impact of climate change in a way that would otherwise not be possible. Consequently, his work has the potential to be really useful in the future, if someone follows up on it. I should also note that his work on slime molds substantially helped the development of Dictyostelium as a model system, which may well be his most important contribution to date.

His marine work was largely on hydrozoans; he did careful taxonomic work and collected extensively in Sagami Bay. This is important for the same reason I gave above, and more specifically because it documents the biota of Sagami Bay, so is a valuable tool for resource management. Again, not super sexy, but important.

I tried a little to figure out what his impact factor or H-index would be, but because he published under a few different names and because many of his publications were in the Japanese-language scientific press, I wasn’t able to come up with anything that I would regard as a reliable metric. I think it is safe to say, however, that he was much more than a hobbyist, and did careful and well-informed research within his discipline. His publications are taken seriously, and his work will probably be referred to for its scientific content for years to come.

Regarding what name he used to publish, some of his publications just say “The Emperor Hirohito,” but he also used a pseudonym, H. Hattori, e.g., Hattori H 1964 Myxomycetes of the Nasu District 1st and 2nd edn (Tokyo: Science Council of Japan)

Unfortunately most of the publications I can find are behind firewalls, but here is a memorial essay that talks a little about his accomplishments.