Waverider

Both port security and piracy are pursuits best taken by waveriders, warriors that conquer the oceans and seas astride a swimming mount. While some of these unorthodox fighters ride entirely aquatic animals, those who live lives away from the coast opt for a more amphibious companion.

Unusual Swimming Mount

When you choose this archetype at 3rd level, you gain the Unusual Mount feature and become the mounted guardian of a giant frog , giant poisonous snake , or reef shark . You also gain proficiency in the Spirited Steed combat tradition if you are not already proficient. When you gain this feature you can choose one of the maneuvers you know and replace it with another maneuver of the same degree or lower from the Spirited Steed tradition. These maneuvers do not allow you to attack with your unusual mount, though you can use such maneuvers as normal on a different mount.

Against the Current

Also at 3rd level, your mount has become hardy. While riding your unusual mount, its AC increases by an amount equal to your proficiency bonus and it gains proficiency in Strength, Dexterity, and Constitution saving throws .

In addition, when your unusual mount takes damage, so long as you are riding it you can spend 1 exertion to reduce the damage taken by 1d10 + your fighter level. Beginning at 15th level, once per round you can reduce damage taken by your mount without spending any exertion.

Fluid Disruption

At 7th level, whether on waves or land, your unusual mount can make enemies vulnerable to attack. You can use a bonus action and spend 1 exertion point to direct your mount to undulate, splash, jump, or otherwise make disorienting movements, distracting creatures within 10 feet of it. Until the end of your next turn, attacks made against these creatures are made with advantage . You can spend one hour during a long rest to familiarize friendly creatures with this tactic, making them immune to the effects of this feature.

Diving Experience

At 10th level, you (and your unusual mount if it is not already able to breathe water) are able to hold your breath for a number of minutes equal to your fighter level and ignore the effects of water pressure. You also gain a swim speed equal to your Speed.

Additionally, your unusual mount can drag a number of Medium or smaller creatures equal to your proficiency bonus through the water without reducing its swim speed. By spending 1 exertion per additional creature, your mount can drag an additional Medium creature (up to a maximum equal to its Strength modifier) for 1 minute. Due to size constraints, a rope can only hold two creatures for each 5 feet of length, unless they are small enough to share a square.

Lessons of the Waves

At 15th level, you have learned well from surviving the dark and dangerous waters. You and your unusual mount gain advantage on saving throws made to resist being paralyzed , stunned , rattled , frightened , or restrained .

Aquatic Mastery

At 18th level, you gain an expertise die to attack and damage rolls while riding your unusual mount in or on the water, and while you and your unusual mount are in or on the water, creatures have disadvantage on attacks against you and your mount.