Here's what I've made, please feel free to comment.  I'm also looking for help on getting it put into DDB properly.  I'm having trouble getting the "if" conditions coded correctly, can't figure it out.  I'm willing to make changes to get it to work within the confines of the subclass homebrew process here.

The archetypal mariner excels at combat above the waves as well as under them.  A mariner is at home sailing across the seas and oceans.  Most have served aboard various ships, either in the employ of a powerful merchant guild, naval armada or even a wealthy patron who owns a fleet of ships.  Famous mariners are equal parts feared and respected among all the sea-faring folks and known among the denizens who live below the depths.

Mariner Features

Bonus Proficiency

When you choose this archetype at 3rd level, you gain proficiency in Vehicles (Water). If you are already proficient in it, you gain proficiency in Navigator’s Tools.  Your proficiency bonus is doubled for any check you make that uses Vehicles (Water). You receive this benefit regardless of the skill proficiency you gain from this feature. [How do I get the Navigator's Tools as a proficiency on the "if" condition of having Vehicles (Water) from another source?]

Born of the Sea

Starting at 3rd level, your mastery of the seas extends below the waves.  You gain a swim speed equal to your speed.  If you already have a swim speed, you instead gain the ability to dash as a bonus action on your turn while swimming.

Sailor’s Gambit

Starting at 3rd level, you can deceive your enemies into letting down their defenses.  As a bonus action, you can perform a feint against one enemy within five feet.  You have advantage on the next weapon attack roll against that enemy.  You can do this a number of times equal to your Charisma modifier and regain all uses after a short or long rest.

Corsair’s Footwork

Starting at 7th level, you have mastered keeping steady on a ship as it’s tossed by the waves.  You gain the following abilities.  You have advantage on any saving throw made to keep from falling prone or against any effect that forces movement.  This does not include teleportation, effects that send you to a different plane, or compelled movement, such as from a fear effect.  Additionally, you can move five feet after you complete an attack action or opportunity attack.  You do not provoke opportunity attacks when taking this move.  This does not grant additional movement when used on your turn.

Aspect of the Storm

Starting at 10th level, you gain the supernatural ability to withstand and channel the most powerful tempests of the seas.  You gain resistance to lightning and thunder damage.  Also, when you hit using your sailor’s gambit, you deal an additional 1d6 lightning or thunder damage (your choice when you hit).

Captain’s Gallantry

Starting at 15th level, your actions inspire your allies, regardless of your rank or station of those who fight with you.  When you use your action surge, up to three allies of your choice within 30 feet that can hear you regain hit points equal to your Charisma modifier (minimum 0) plus half your fighter level.  Additionally, your allies may use their reaction to move up to 15 feet without provoking attacks of opportunity.

Storm Prodigy

Starting at 18th level, your mastery of the seas is unequaled.  You are immune to lightning and thunder damage, and when you hit using your sailor’s gambit, the additional damage is increased to 2d6 lightning or thunder damage (your choice when you hit).