Broken Splitscreen Water Visuals
In splitscreen, only the host is able to properly see through water. For other players water looks like a solid block regardless of biome. Enchantments such as Aqua Affinity do not affect the visuals of the water and potions such as Night Vision also do nothing to mitigate this bug. While underwater the issue remains and visibility is severely reduced. Enchantments and potions fail to help. The issue affects all splitscreen players other than the host regardless of the amount of splitscreen players.
Created Issue:
Broken Splitscreen Water Visuals
In splitscreen, only the host is able to properly see through water. For other players water looks like a solid block regardless of biome. Enchantments such as Aqua Affinity do not affect the visuals of the water and potions such as Night Vision also do nothing to mitigate this bug. While underwater the issue remains and visibility is severely reduced. Enchantments and potions fail to help. The issue affects all splitscreen players other than the host regardless of the amount of splitscreen players.

I can confirm this is an issue on Xbox splitscreen. It can be pretty soul destroying working on underwater builds.
Also, this post indicates it's an issue on Switch too:
https://www.reddit.com/r/Minecraft/comments/a8gynj/trying_to_play_multiplayer_but_cant_see/
Duplicate of
MCPE-39539, please search before reporting.