Well, we've explained why we are using crossable straits many times before, so if someone thinks that we are just assuming an army could 'walk across' a strait then that is just plain wrong. It's an abstraction of what really happened. Like an abstraction of a bridge across the Nile or lower Danube (those things didn't really exist at any time - they just stand for small ferries). For small straits where a real fleet (what our ships represent) wasn't needed, but instead small local ships like a ferry would suffice, then we are ok with having armies "cross" these straits.