The Italian ships are already there, and Bisko blows up the onsen around them.
The Admiral might had gotten that voucher from the Italian ships (maybe in exchange of cancelling their prison term), they probably ratcheted up quite the amount of reward points from all the time they spent there.
Knowing Ido's base, other shipgirls will already be at the same onsen ruining Graf's plan for lovey-dovey onsen trip.
Alternatively, Graf and Admiral enjoy the trip, and nothing goes wrong. Although they become increasingly paranoid and are constantly expecting something to go wrong.
Hmm... Not bad at all!W-Well now... An onsen trip for two, is it?She certainly seems pleased...It's important to properly take time for work and vacation alike, after all!Plus, we haven't had any alone-time recently...