Base Class: Ranger
The marine thrives along coasts and in the water. This ranger subclass specializes in shaping/controlling/moving through the rough waters in the vast oceans of the world. These rangers usually have some deeper connection to certain bodies of water of which they have spent a considerable time in and around.
Marine Magic
Starting at 3rd level, you learn an additional spell when you reach certain levels in this class, as shown in the Marine Spells table. The spell counts as a ranger spell for you, but it doesn’t count against the number of ranger spells you know.
Clear Waters
The depths can be very dangerous and as such at 3rd level Marines gain a sense of clarity and being in the moment when they enter the water. Because of this Marines make all wisdom and intelligence checks at advantage while in the water.
Blood in the Water
Starting at 3rd level when you hit a foe with a weapon attack and they have less than half of their max HP you inflict 1d4 of extra damage, the same type as your weapon causes. This bonus increases to 1d6 at 11th level.
Amphibian
Starting at 7th level after so much time spent in the water the Marine starts to acclimatize further to fit the environment. If they are not already the Marine gets proficiency in the athletics skill and in constitution saving throws.
Fluid Motion
Spending time in the tides and currents of the world's oceans leaves an impression on Marines. When they reach 11th level they learn to become especially evasive in struggles and their experience gives them a grace during combat. More specifically Marines learn the whirlwind attack option and gain advantage when trying to defend against or escape from being grappled or restrained (this also allows the Marine to use their dexterity modifier instead of strength).
Protective Tides
Starting at 15th level due to the ever increasing affinity with water and magic the Marine can now use the shield spell an amount of times per long rest equal to their proficiency bonus. They regain all expended uses when finishing a long rest. This version of the shield spell is made of water instead of arcane energy but the spell is cast in the same way.