Looks like underwater endurance training that Tenryuu was doing and as per usual Tatsuta doesn't tell Kiso about it in order to troll her older sister.

Come now, Kiso. As a fellow chuuni, surely you'd know a ninja training when you see one??

God Damn It Tatsuta!

It says something about how casual the long timers are with the admiral, that their idea of chatting with him is meeting in the bath.

I kinda find it amusing that the artist deliberately hid Tenryuu's eye despite not wearing her patch.

Speaking of which, it would be interesting to find out what Tenryuu's hiding in her left eye since Kiso has heterochromia (tho Furutaka would technically count but she aint hiding that fact)