First and easiest: How can i disable humanoid floating up automatically?
Second: Humanoid swimming seems to ignore swimming up/down up to a certain angle.
Some visuals (pay attention to the command line):
Sorry for low fps. 
move Vector3 (0,0,-1)
move Vector3 (0,-.25,-1)
move Vector3 (0,-.5,-1)
Can i (and if how?) make it so it transitions smoothly?
You could probably mess around with the physical properties of the character like Density if that doesn’t work theres a few methods to disabling swimming entirely and writing your own swimming system
https://scriptinghelpers.org/questions/32907/swimming-how-to-disable
Found this.
Allriiight, this could work.
I didn’t know you could disable it.
nope
Still open to other options.
HumanoidRootPart seems to resist all body movers.
Decided to rewrite the ENTIRE movement system to not relly on :Move().
Topic closed, but you can still answear.