Something I like about the modular nature of D&D 5e subclasses is their ability to explore niche or situational archetypes. The great thing about subclasses is, nothing is lost by having them available! Nobody is forcing you to play one if it’s a bad fit for your game.
As the library of official subclasses gets larger and larger, I think there’s more room to explore and embrace these niche characters types. An aquatic-focused Druid, for instance, is a poor choice for most campaigns, but in an adventure where it can really shine, it can be a ton of fun to play.
Druid Circle of the Depths
The Circle of the Depths is made up of Druids that share a greater affinity to the sea than the land. These druids protect the oceans, rivers and lakes in the same manner than another Circle may safeguard a forest. Swift and sure, members of the Circle of the Depths are at their strongest near a water source. It is from these natural waters that they draw the power to give life, protect, and defend, and their most powerful bestial forms are those that inhabit the deep waters of the world.
Starting at 2nd level, you gain a Swim Speed equal to your base speed.
Mantle of Waves
Also at 2nd Level, you can protect yourself by producing a roiling current of water around your body. As a bonus action, you can adopt a Mantle of Waves for 1 minute. While this effect is active, if you are not wearing armor, your armor class equals 10 + your Dexterity modifier + your Wisdom modifier. Once you use this feature, you can’t use it again until you finish a short rest.
Child of the Sea
Starting at 6th level, you can breathe underwater. In addition, when you use Wild Shape you can transform into a beast with a challenge rating as high as your druid level divided by 3, rounded down, as long as that creature has a swim speed and no other movement speed listed.
At 10th level, you can cast Water Breathing, Water Walk, or Control Water once without expending a Spell Slot. Once you use this feature, you can’t use it again until you finish a short rest.
Font of Life
At 14th level, your swim speed is increased to double your base speed. In addition, you heal 1 HP every round when submerged in water.