Base Class: Ranger
Whalers are most adept at hunting prey on the open sea. Few know what true evil lurks in the bottom of the seas better than the whaler. Even fewer are willing to believe the tales.
Whaler Magic
Starting at 3rd level, you learn an additional spell when you reach certain levels in this class, as shown in the Whaler Spells table. The spell counts as a ranger spell for you, but it doesn’t count against the number of ranger spells you know.
Plunge Into The Deepness
You have spent so much time at sea you are no longer find yourself flailing when off your ship. Starting at 3rd Level, you gain a Swim speed equal to your base walking speed.
Thoughts of the Leviathan
Starting at 3rd level, you can focus your ire on one foe, increasing the harm you inflict on it. As a bonus action, you designate one creature you can see within 60 feet of you as the target of this feature. The first time each turn that you hit that target with a weapon attack, it takes an extra 1d6 damage from the weapon.
Facing the Inscrutable
At 7th level, you gain extra resilience against your prey’s assaults on your mind and body. Whenever this target forces you to make a saving throw and whenever you make an ability check to escape that target’s grapple, add 1d6 to your roll.
Relentless Pursuit
At 11th level, with your bounty in your sights, you are undeterred by a missed attack. Once on each of your turns when you miss with a weapon attack, you can make another weapon attack as part of the same action.
Last Breath
At 15th level, your sheer determination and drive to see a foe slain sustains you when you would otherwise should perish. When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead.
You can’t use this feature again until you finish a long rest.

I wanted to say that I like the direction of this subclass for a sea ranger. A few notes however, I would really suggest adding the spell list from Whaler magic to the description. That way players can see what they are getting in to. I like create/destroy water but purify food and drink would be a nifty alternative. Lastly, the Misty Step spell comes up as a 5th level spell not 2nd which is intended.
Again, a lot of potential in this subclass with just a touch of changes.