Are there any ways to increase a PC's swim speed, magical or otherwise?

Question

Are there any ways to increase a Player Character's swim speed? Said ways can be magical or otherwise. This is mainly for a changeling rogue.

Answer 1

Gain a swimming speed equal to your walking speed through a spell, magic item, or multiclassing. Then increase your walking speed.

There are a few ways to gain a swim speed equal to your walking speed. If you can obtain and attune to a Ring of Water Elemental Command and then kill a water elemental, you gain a swim speed equal to your walking speed (among other benefits).

Gloves of Swimming and Climbing also make it so that swimming costs no extra movement, meaning you can use your walking speed when swimming.

Alternatively, the Freedom of Movement spell makes it so that

being underwater imposes no penalties on the target’s movement

which effectively means you can move up to your walking speed while under water if you are under the effect of that spell.

The Alter Self spell can also grant you a swim speed equal to your walking speed.

Path of the Beast Barbarians can give themselves a swim speed equal to their walking speed starting at level 6.

Another way is through the "Deft Explorer" optional Ranger feature presented in Tasha's Cauldron of Everything. At 6th level, the Rover sub-feature grants a 5 foot walking speed increase and a swim speed equal to your walking speed.

Another way to gain a swim speed equal to your walking speed is through the "Gift of the Depths" Eldritch Invocation. This requires 5 levels of Warlock.

Once you have a swim speed equal to your walking speed, you can increase your walking speed through the Mobile feat, the Longstrider or Haste spell, multiclassing into Monk (at least 2 levels), or Barbarian (5 levels), or through magical items like Boots of Speed.

Or gain a flat swim speed through multiclassing or a magic item.

Storm Herald Barbarians (specifically of the sea) gain a 30 ft swim speed at level 6.

Fathomless Patron warlocks gain a 40 ft swim speed at level 1.

The Cloak of the Manta Ray grants a 60 ft swim speed.

The Ring of Swimming grants a 40 ft swim speed.

The Superior Mobility feature gained at level 9 by the Scout subclass of Rogue adds 10 feet to your walking speed, as well as your swim speed if you have one, making it relevant whether your swim speed is equal to your walking speed or a separate constant.

Or transform into something better at swimming than you are.

This can be done through a Druid's wild shape if you're at least level 4 as a Druid, the Polymorph spell, the Animal Shapes spell, the True Polymorph spell, or the Shapechange spell. If you're a Rogue, you may need an ally to cast one of these spells on you.

Answer 2

Polymorph, wild shape and various items

The polymorph spell can be used to change into any animal with a DC less than or equal to your total levels.

If you are a high enough level druid you can wild shape into a creature with a swim speed.

Otherwise there are a number of magical items such as:

1. Apparatus of the crab
2. Cloak of the manta ray
3. Gloves of swimming and climbing
4. Ring of water elemental command
5. Ring of swimming

You could also, presumably, summon a creature with a swim speed and have it carry you through the water.