They use boyish language (boku is the masculine reference to self, and is less elegant than watashi), sometimes boyish looks (see Suiseiseki), and usually display an energetic and outgoing or boisterous personality unlike the traditional yamato nadeshiko demureness that was seen as the ideal for feminine behaviors.
Every one of the shipgirls in the comic is a bokkuko, which is why it's titled "tlThe 6th boku".
And Matsukaze (the shipgirl, not that dog admiral) is looking like the type of shipgirl that I like.
You "could" also add Tenryuu to the mix with her usage of "ore" but she's mostly seen as a Chuuni though than a Tomboy.
I trust you've got my back.
She uses masculine pronoun bokuA...A BOKUKKOOOOOOOO!!It can't be - you're a...!What!?Impossible...4th of the Kamikaze-class destroyers, Matsukaze.gulp..