Identify anime with submarine under aerial attack
Thanks to a very diligent redditor I was able to find it.



海底大戦争 愛の20,000マイル
The Great Navy Battle: 20,000 Miles of Love

Released by Tatsonoku in 1981. Apparently it was dubbed by Harmony Gold as Undersea Encounter but was never released for home video, which probably explains why there's almost no trace of it in the web except for a few cover images and brief synopses (mostly in Japanese).