Underwater defenses for surface ship navy?

Evil king, evil kingdom, mostly humanoid, land based, sends a naval surface fleet to attack.

The party is high enough level that they might try to stop them directly. Which is fine by me, set up the pins so the party can knock em down if they want. The party has also acquired magic items and other means so that everyone can operate underwater. So that will -probably- be their path to attack.

Im looking for ideas for ways the bad guys would defend their surface fleet from underwater approach and attack.  If it were a modern fleet, it would be an attack sub chasing off any underwater threats.

Maybe they hire underwater species as mercenaries?  Sahuagin?  Steal the navy's trained dolphin idea, but make them deadlier? I dunno.  The attackers are evil, so maybe something evil.  I dont want it to be an impenatrable defense. Its going to be a tactical group of ships, sailors, and soldiers. So, they probably dont have a trained kracken.

But the ideas ive come up with are uninspiring. 

Ideas?

Well, the fleet might just not think to protect the underside of their ships, unless they know the players have the capability to go underwater.

• Ironclads, provides surface and below water protection
• Underwater mercenaries, probably sahuagin like you said, paid with the spoils they pilfer
• Bottom of the ship equipped with an autonomous defense. Erupting sigils that don't damage the ship, only intruders, nets that catch and drown, spring-loaded spikes on the outside

I was recently reading a novel called Steel Gods which had a bunch of fighting between humans with roughly 18th-century naval capabilities and merfolk who ride on tamed sharks. One of the things the humans did was cover the undersides of their ships' hulls in sharp spikes that prevented underwater attackers from getting close. They also threw lots of raw meat into the water in the hopes of riling up the sharks and making them hard to control.

It's pretty hard to have an effective navy in D&D-ish fantasy without having good underwater capabilities, so either they're gambling on surprise or they've got aquatic or amphibious auxiliaries. In the latter case, I'd just go through some sort of list of evil aquatic monsters (adjust based on sources you have access to) and find something you like.

Heh. Yeah i was def stuck in a rut. Some good ideas here. 

Navy has powerful creature under control 

This one is a little convoluted, but here goes. Maybe each ship has a caster and a couple scrolls of water breathing. And a group of specially trained sailors to deal with the threats. They fit ropes along the underside of the hull and use a caribiner to attach themselves when the fighting starts. Caster gives them the water breathing and the run around under the ship, allowing the ropes to hold them so they don’t need to swim to keep up with the moving boat. Let there be some slack so they can move around a bit away from their ropes. All equipped with piercing weapons, since they’re trained for underwater fights. Maybe a wizard who makes sure to not have fire spells prepped. Heck, maybe mount a ballista down there. The obvious weakness is the ropes, so either you make them steel cable and so harder to break, or you let the PCs figure out they can try to cut the ropes. 
Sahugin riding sharks is cool and all, but they’re pretty low CR. Something like this you can re-skin lots of bad guys to be the soldiers.

Electrical fields

Nets

Sahuagin - they always make good bad guys.

Depth charges

Going down the evil route:

Blood. The navy has sacrifices which pours blood into the water at the first sign of underwater attack. The blood attracts sharks (and worse). They continuously feed them, to keep them close.

Antimagic depth-charges. Assuming there's no means by which to go underwater non-magically, dropping something into the water which can end such spells or suppress magic items would be highly effective.

Poison. If their foes breathe water, make the water unbreathable.

Some sort of grappling barnacles on the underside of the ship (like miniature ropers) which hold such invaders to the hull. Have a few coral structures on the underside which look like people, to emphasise.

Oil & fire. They can't come to the surface if the surface is fire.

Cold. the arctic has ice pilalrs that form from the surface and kill things underneath. You could have them have something similar.

A great bell which deals thunder damage to things underwater.

For maximum interaction, I would have them have numerous ships which sort of telegraph their purpose -make them distunguishable, so you might say "a dark vessel, the boards stained black, with metal funnels hanging from the walls like cannons pointed at the deep". This is the ship which drops blood in the water - it's stained from the blood, the funnels drop blood. when they see it again, they will know what to expect. They might opt to target it, or to sink the other ships so it summons sharks to kill their own sailors.

If each ship is distinct and does its own thing, the party can make plans around it.

Depending on how long you plan on this assault lasting in the campaign, you can have the first effort be extremely likely to succeed - the enemy has no defences against underwater. Then, the next time, they have some defences. the next, they have a lot of defences. Make the enemy adapt to the party - I have a villain in mine who made a helm of legendary resistance which was in direct response to the party using Hold Monster on him - essentially, it gave him advantage against mind control and one legendary resistance. He telegraphed it so they didn't waste it twice - the first time she tried, I said "he's going to roll with advantage", and then he locked eyes with her, tapped his new helmet, and said "oh, no, not this time." - now the party know not to try that spell on him!

An antimagic/antilife spell on the hull.

The fleet has man-o-war type jellyfish around each of the important vessels. 

They employ cormorant type monsters as air support.

Casters have learned to cast "Steamball" instead of Fireball.

Apparatus of Kwalish!

Use intelligent Seals as S.E.A.L.s

Water Elementals