Hi there! And, thanks for the feedback.
By default, swimming works by holding jump (for slow) or charge (for fast) and aiming with either of the two gamepad joysticks, a keyboard’s arrow keys, or a mouse (with inversion, look input coupling, and control binding options in the settings menu). That sounds like what you’re describing. So, perhaps there was just some misunderstanding with how the controls work.
Did you find the portal to and try The Scrapyard level? If not, there’s some challenge to be found there, as it represents a later stage of the game. Although, it will likely see a bit of a rework at some point to improve gameplay flow and polish some details. The demo’s starting area, being just the first hub world, is relatively simple.
In any case, it’s nice to hear that you enjoyed it. More levels to come in the final version, of course.