A Trident with Impaling V doesn't deal extra damage to drowned, even if they are aquatic mobs.
As you can see on the video I attached, the drowned dies after 3 hits no matter if it was killed with a netherite sword, a thrown unenchanted trident or a thrown trident with Impaling V.

An Elder Guardian needs 10 hits with netherite sword, 10 hits by a thrown trident or 4 hits from a thrown Impaling V trident in order to die, as it is an aquatic mob. 

Netherite swords deals 8 Melee damage;
Normal Trident deals 8 Ranged damage;
Trident with Impaling V deals 20.5 Ranged damage (to aquatic mobs).

What I expected to happen was:
The drowned would have been killed with only one ranged trident hit.

What actually happened was:
The drowned died after 3 ranged trident hits, the same as using a normal trident.

Steps to Reproduce:
1. Spawn a drowned and kill him with a trident (thrown)
2. Then spawn another drowned and kill him with an Impaling V Trident (thrown)
3. Notice how the drowned died after 3 hits in both cases
4. Repeat step 1 and 2 with a guardian or elder guardian
5. Notice how this time the normal Trident needed more hits than an Impaling V one.