Perhaps not due to it being a speed effect (even though water does modify player speed). Still, this is inconsistent.
Linked issues
discovered while testing
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migrated
Brevort
Confirmed for 21w06a. My guess is intended, but that's not up to me to decide.
Avoma
Can confirm in 1.16.5.
Felix Jones
Can confirm. However the normal FOV setting doesn’t affect the hand and the item that you are holding in your hand, while being underwater does. So this may be intended.