Deep-Sea Sentinel

Deep-Sea Sentinels watch over the shores of islands and continents, hunting down the horrors that emerge from the black depths of the seas. They often explore the oceans and seas to kill the corrupted and abhorrent abominations that lurk within the abyssal waters so that the surface can remain be safe and remain ignorant of the nightmares that exist in the chasmic waters.

Tidal Stalker

Starting at 3rd level, you are now a Deep-Sea Sentinel, and hunt the monsters that lurk within the far reaches of the Oceans that threaten to destroy coastal areas. As such, you need to fight those creatures in their own lair. You also learn to use the waves to make sure the tides are on your side. You gain a swimming speed of equal to your walking speed and can breathe under water.

Additionally, once per turn, when you attack an enemy with a weapon attack, you can choose to force the target to make a Strength Save(the DC equals your Spell Save). On a failed save, the creature is pushed 15 feet away from you by the waves. 

Hunter of the Depths

Starting at 3rd level, you have become adept at stalking the horrors of the abyssal oceans, and are good at tracking them. You have also learnt the languages of the people of the deep in order to aid them when in need. You can now speak, read and write Primordial. Additionally, Underwater becomes favoured terrain for you, and does not count against your current number of favoured terrain.

Benthic Pressure

At level 7, you have spent lots of time underwater and have experienced the herculean pressure of the water. Due to this, you have learnt to magically replicate the crushing pressure of the deep waters to punish your foes in a split second.

Abyssal Pressure: When a creature moves within 15 feet of you, you can use your reaction to force it to make a Constitution saving throw. On a failed save, a creature takes 1d8 bludgeoning damage and its speed is halved until the start of your next turn, and takes half as much damage on a successful save.

The damage dealt increases to 2d8 at 11th level.

Neptunian Endurance

 At 11th level, your explorations and expeditions through the watery depths has inured you to the dangers that lurk there. You gain resistance to piercing, bludgeoning and slashing damage dealt from non-magical weapons, and you are resistant to acid and cold damage.

Swift Currents

By 15th level, you are so used to the buffeting strength of the depths that little can deter you, whether physical or magical. Your movement speed doubles and you become immune to being Grappled, Paralyzed and Restrained, and your movement speed cannot be reduced to 0 by magical means. Additionally, any opportunity attack made against you is made with disadvantage.