Tidying my flat I found my copy of The Anime Encyclopedia, and yes it does have an entry on that Tatsunoko Pro title I’ve been whittling on about these last two weeks.
Apparently it had US dub from Harmony Gold under the name Underwater Encounter, but that doesn’t turn much up either. It was filmed on 35mm and took 2 years to animate. Beyond that there wasn’t much more that I gleaned from sites I quoted before. It’s odd that something that Tatsunoko took so much effort to make has near disappeared from discussion, I wonder if it had had a proper theatrical release it wouldn’t be so obscure.