Because a living submarine is bigger than you and has built-in weapons. (Screenshots are from the anime Arpeggio of Blue Steel, which is about a bunch of superpowered and essentially living ships showing up and imposing a global blockade on humanity so that sea trade can't happen. They also shoot down all the satellites and jam all forms of long-ranged communication.)
And yes, the white/blue-haired girl is a submarine. (Her "real" body is just offshore, which is about twelve feet away in this case.)
Bonus reason for not picking a fight with a living submarine: There's a chance they'll be good at hacking.
(Incidentally, the reason this post is in the "super robot junk" label is because I've been watching it for the same reasons I watch super robot and mecha series*, and "super robot junk" is definitely my most catch-all category for that sort of thing.)
*Stuff blowing up plus cool technology porn that doesn't feel the need to make perfect sense.