sub in this context means literally, physically below (or "inside" can also could for the sea, given in English this is the idiom) - but it can also mean small,

sub in this context means literally, physically below (or "inside" can also could for the sea, given in English this is the idiom) - but it can also mean small, lesser, or diminutive - it is far from unambiguous. Marine can mean the sea, but also things associated with the sea. Nearly all words are somewhat metaphorical, idiomatic - we just learn what those idioms are.