I have a society of merpeople that have developed sonar in order to hunt, gather resources, and in some instances communicate. They use organically created weaponry (sharpened coral, animal incisor daggers, turtle shell shields, etc.) and live in woven seaweed fortresses. A rift has been created, and the society has split, leaving two groups of merpeople about to head into war over the organic and natural resources (Prime seaweed for building, excellent coral flexibility in this area, the water currents funnel the fish juuuust right, etc.). I was thinking on how to beef up those defenses, when I hit a block. My merpeople live ~150 meters down, and although they can utilize photosynthesis, it is not a necessity to them. Most of their structures are either built in as part of a kelp forest, or spread out into a hut city of sorts. There is a stigma about caves in their world being cursed by the god of water, so holing up (in some cases literally), is out of the question. They have the ability to burrow, but mining through stone is only done for the rich and the powerful due to its difficulty, and so is not commonly done. Digging is easily done provided the ground is soft enough for a coral shovel to penetrate.
If they have sonar, then there would never be a surprise attack that would gain the advantage on the enemy, and directional warfare would probably be akin to space warfare, in that an attack could come from any direction.
My flaw was in first thinking about the defenses, but I couldn't come up with a reason to not create essentially a bubble of seaweed or shells (think the part in 300 where they have the shield turtle), and just go on with their daily lives using sonar to say, "Oh hey, the enemy is over there".
My question is thus: How would warfare be conducted in an underwater environment? Is there a way to gain the upper hand at the start of a battle?
I'm looking for something like an attack of some sort (blitzkrieg, sneak attack, etc.), but intelligence, espionage, and sabotage might work as well, provided the actual reason for the wars (resources) would remain usable.