When I saw Mari I liked it the most. I also think Mara works. It means the sea in Irish so has the theme you’re looking for and easy to pronounce in both languages. I like how the double a in ‘Mara’ contrasts the sound of the vowels in the boys’ names. Congratulations and enjoy selecting!
[name_u]Ocean[/name_u] related:
I would say Umiko, [name_f]Umi[/name_f] is a word for sea and ko means child and is used is feminine names.
Mizu which means water.
[name_f]Hana[/name_f], works really well in both [name_f]English[/name_f] and Japanese.
I also love :
Midoriko - [name_u]Green[/name_u] child
Kanna - god of nothing? It sounds pretty.
Kagami - mirror
Ryuko - dragon child (?)
[name_f]Megumi[/name_f] - blessing (?)
[name_m]Botan[/name_m] - [name_f]Peony[/name_f]
Kocho - butterfly
[name_f]Hitomi[/name_f] - pupil (eyes)?