Fishermaster

You wanted to swim, swim and swim more. This subclass is right for you. You want to Dive? You don't want to die. Fish, Whales and food? All of this can be yours! You can hold on your breath and control Water. You don't have any friends? No Problem! You can search you new friends in water! If you choose this subclass at Level three you will never be alone any more!

Friendly Water

You´re swim-speed is doubled. If you haven´t any swim speed it´s your normal movement speed.

If you can't hold on your breath any longer, you can use your action and to double the time you can hold on your breathe.

When you're in Water you make + your dexterity- or you strength-modifier damage.

You can also speak Aquan.

Waterarms

You can extend your arms with water, which are as long as your wisdom-modifier in meters. You can do everything with th Arms you can do with normal Arms. 

At Level 15 the reach of the Arms is your wisdom- and your constitution-modifier in Meters.

Creatures of the Sea

You can summon Monsters, which normally live in water, which challenge Levels aren't higher than your proficiency-bonus. When they normally can only breathe under Water, they can breathe air in the time their sumon. When they only have a swim-speed, they swim in the air like they do in water. You can control them with words in Aquan with a bonus-action. 

Waterworld

You give up a spell slot to teleport you and a number of creatures that match the level of abandoned spell slot, in you're own dimension which is full of Water and Islands. You can teleport you and the other creatures back to the place you had teleported to waterworld before and you can leave the others in Waterworld, when you want to.