Finally, the gang is back together again!
Although I don't quite get the speech bubbles exactly here, is it about Mitori unable to swim for some reason or what?

I think Mitori was being wary because Yuugi is after all an oni. Probably intended to beat her, but Yuugi took it positively and asked if she wanted a swimming competition.

IronFly said:

Finally, the gang is back together again!
Although I don't quite get the speech bubbles exactly here, is it about Mitori unable to swim for some reason or what?

Pretty sure Yuugi was basically asking 'oh, where you swimmin' over there'? To which Mitori felt like doing a lot of denying to Yuugi. Physically, with that sign, and as hard and often as possible (you know, for all the good that would do against an oni).

Little Kisume blowing like a grampus...

I do love Yuugi's laid-back "ah, that was a nice jog" look.