[Well, Undyne will go ahead and make him even more uncomfortable by not even reacting to those puns. Instead, she just says really quickly:]

No!

[And then she covers her mouth for a second before putting on a huge strained smile.]

I mean uh - whaaaat, no way could you come up with a better reason than mine! Is that a challenge!! Anyway, you shouldn't make it too complicated, he'll probably catch you. Just use a simple reason. Like mine. But only because it's the best fake reason and way better than anything you can come up with!! Don't insult my pride!!

Unless you actually think it won't work. In which case, you do you. He likes you better, I have complete and total faith in you. Because this is your plan. That you should just make up as you go along. Without any input from anyone else.

But my reason is. Definitely super cool and great and better than yours even though I just made it up this second and youshouldtotallyuseit. Trust me, it's not fishy at all! Water you so worried about?! Where there's a gill there's a way! Hahaha, get it?!

[And then she. Gives him double finger-guns. That alternate universe theory sure is looking better and better...]