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