Ido version: 1.0.4 (1.12.2)
Forge 2838

Video proof

Also, could you nerf player's vertical movement underwater? It feels like I can go from the water surface to the bottom of the ocean in 2 seconds

oops, my bad, had it private
try it now

I'm doing a list of my installed mods to see if you can find any incompatible mods

Video is unavailable for me

Never seen an issue like this before, very odd. Yah get me that list of mods so I can check

Try removing carryon and see if that fixes anything

What mods do you have that change the player in any way

Removing CarryOn did not fix the issue

Directly, I think Cosmetic Armor (replaces the worn armor with something else at your choice)

Even after removing Cosmetic Armor I couldn't fix it

I'm talking about things that replace the playermodel

I don't have any mods that do this, only mods that add extra rendering (eg: Baubles and such)

gonna remove all the mods and try slowly readding them back until the issue comes back

Found the culprit, it was Better Portals (dunno why and dunno what it does with the player rendering)

So, now that the culprit was found, Is there a way to fix it?

I never found an issue with swimming or crawling with betterportals, but it may be do to the fact that betterportals renders the multiplayer version of the player on singleplayer. Are you using betterportals 2 or 1?

0.2.4 (the latest one)

Ah i see the exact issue here. Because of the way BetterPortals handles rendering with the player, I cannot just simply do a fix on this. This is up to him to try and fix it from what it seems, and also the fact that BetterPortals is way out of my league as its made using kotlin, of which I know none of. I will make an issue there explaining the issue.

Johni0702/BetterPortals#200

I have made the issue here

Also I will try to see what I can do on the front of changing the way the player moves in water vertically

Ok, thanks :)
Currently the vertical movement feels way off in comparison with 1.13/14, the player rises and dives way too fast