So, my suggestion is 2.5-fold:

1. Drowned can now mine blocks underwater, using their trident. It's slow, but they will use it as a breaching behaviour.
2. Zombies have ~2 modificitations
   2a. They can dig underground (They will only dig to target players, and the chance to be able to dig below sea level is like 75%, but only if they can dig to begin with! (They can't dig in water though. But, they can have the drowned do that...)
   2b. They will work with drowned. The drowned can dig them a tunnel to more easily reach the inside and stay land-breathing! Creepers will also work with them to breach that tunnel!

note: 2a is the easiest, 1 is the second easiest, 2b is a bit overcomplicated

This seems way too overcomplicated

yeah... the first part (below sea level) is probally easy. Same with the no digging in water.

The drowned mechanics are where it gets complex...

Drowneds can already mine blocks underwater as they are Zombies.
Making them work together is something I don't want to do. Zombies are brainless, they don't work together