The bug

It looks like you are diving up faster in downwards flowing water when there are non-opaque solid blocks next to you.

How to reproduce

1. Create a one wide hole inside an opaque block and put water at the top

   /fill ~1 ~ ~1 ~3 ~5 ~3 sandstone hollow
   /setblock ~2 ~5 ~2 flowing_water
   

2. Go to the bottom of the hole and dive up
3. Repeat the steps for a non-opaque solid block

   /fill ~1 ~ ~1 ~3 ~5 ~3 glass hollow
   /setblock ~2 ~5 ~2 flowing_water
   

   → You dove up faster