I though about 'submarine', but couldn't see how 'sub' meant 'junior'. I suppose that it could be short for 'subordinate', which could just about be equivalent to 'junior' i.e. the office junior might be your subordinate, but that really is tenuous...
I agree but it was the only answer I could come up with! I was trying to think of the name of the young of water creatures but too tired to think after working hard in the garden.