The bug
The new fish mob that was added has 20 health. This is more than a squid which leads me to believe it is a bug.
How to reproduce
Place down a fish (cod in this case)
Run
/data get entity @e[type=minecraft:cod_mob,limit=1,sort=nearest]
→ The Health tag is20.0f
Can confirm this, it takes exactly 20 hits to kill a fish, the same as for a creeper.