I don't know how to explain exactly but the Shinkaisei-kan always speak with the Katakana writting. So I was surprised to see that they can speak normally, but they explained here why.
The katakana is to indicate (we assume) that they are not speaking normally at all (they do in the game, too). Going by the voice clips, they are quite 'unnatural' sounding, like robots. Or, you know, creatures from the deep abyss.
Katakana in most writing is used to indicate "Foreign" speech and words. At least normally. In the case of most comical writing, Katakana is used even for normal japanese speech but as an indicator that the characters aren't speaking normally.
The closest english equivalent I can say is Megatokyo's use of parenthesis to indicate japanese speech as opposed to english.(obviously still in english so the readers can understand what is being said).
They practiced so that they can talk like normal folks of the town. And on the note, most of the works on Kancolle depict that the Shinkaisei-kan talk with katakana, a foreign/machine-like voice.
Don't call me 'Wo-chan'!We gonna have a long talk later, dumbass.And now, it is time for our efforts to bear fruit!The fuck you goin' on about, startin' off like that? We gotta worry the most about you, little miss talkative.Seaport-chan!That's right!Okay! It's check time!
Our mission is?!D-Dang... Such a foul mouth there, Wo-chan...To adapt ourselves for human society, we shed sweat, blood and tears!...investigation and infiltration of the enemy territory...Water strider is reeeed
