From the table in post 20 (thank you, Mr. Shinpachi):
Data Base: Japanese Aircraft Engines
https://ww2aircraft.net/forum/attachments/3-e-jpg.402085/
Max. boost pressure is only +300 mm Hg (1060 mm Hg or 1413 milibar absolute pressure, +5.8 psi boost pressure, 41.73 in Hg manifold pressure). It's the same, as in engine Ha-115 (Sakae 21).
From this I believe, that even with this engine water injection was not used. Fuel octane number is was sufficient.