Just have both grab hold of the tub without climbing in. While there may not be room in the tub for three or it may not support the weight of three surely it can help two swimmers float. There's no way I would abandon either of them. That's the answer you were looking for Yukarin?
Bzzzzt!Time's up, Chen.That's an old reference, Yukari-sama...We understand, don't think about it any more....you see Ran and me about to drown.If I told you you had to choose one, what would you do?When suddenly...Whaaat!?Ah, y- Oh, oh my...R-...But...What would you do?...-shama?Say you were crossing a river in a tub, Chen.You'll overheat.Cheeen.... Y-Yukari...There's only enough space for one other person in the tub.Ran-shama too...That won't do.Yes, ma'am!CheeenShssss
PssssshAllll right, then, here's a question from Miss "How Embarrassing" Yukari! Refers to Maicchingu Machiko-Sensei, a show/manga from the early '80s.Fret
Dither...Toinnng