Since we're doing etymology, take a look at "military". It derives from miles, militis, a soldier.

Nautical or navy arise from nauta, nautae, a sailor.

Why did the word for a coin give us the word for "soldier"? Because back in the day, a soldier was a warrior who accepted his recompense (salary, from the soldier's ration of salt) from the state through his commander; all the other warriors got theirs in the form of loot.

In the British tradition, the personnel of the Navy remained ad hoc far longer than that of the Army -- well up into the 19th century, in fact -- hence the whole thing of the press gang. Soldiers did go to sea, but their status was so different that the language quickly acquired a special word for them: Marines. And they are not sailors, and never have been. (Just ask one.)