Hey there, just thought I'd add my two cents and agree with what seems to be the consensus so far: the swim feature needs major improvement.
I was blown away by how otherwise incredible the Toybox Demo experience was. It was as thought my wildest dreams had come true! The game felt just like Banjo-Tooie in just about every way, and I mean that in the best sense possible. It had all the wonderful traits that I loved when I played Banjo-Kazooie and Tooie as a kid and gave me a rush of nostalgia combined with excitement for the future.
The one aspect that was jarringly non-excellent was the swimming function. I found it impossible to simply swim in a straight line underwater. No matter what I did, the controls were horrendously imprecise, non-responsive, and seemed to constantly result in me swimming directly down with my face bumping into the floor. To be honest, it is easier to swim forward underneath actual water than the water in the game. The function just did not work and was deeply frustrating, to the point that if I had to play a level that required skillful navigation underwater, I would probably rage quit in frustration.
I'm writing today in the hopes that the developers are reading these posts and can actually make improvements to the game before it's ready for release. The swimming feature was the only blemish on the otherwise amazing experience I had in the demo. It's the only criticism I have, but it's a major point for me because of how frustratingly ineffective it was. I would be incredibly happy with a return to Kazooie/Tooie-style swimming, and I think it would make the experience perfect.