To get a swim speed equal to your walking speed in DnD 5e, you need use a spell, get a class/subclass feature, get a racial feature, or obtain a magic item. Here are more details on each of those methods:

Spells:

1. Freedom of Movement. Removes penalties on your movement while underwater.

2. Alter Self. Aquatic Adaptation grants swimming speed equal to your walking speed.

3. Polymorph. Many beasts have a swim speed that meets or exceeds their walking speed.

4. True Polymorph. Same as Polymorph.

5. Shapechange. Many creatures have a swim speed that meets or exceeds their walking speed.

Class/Subclass Features:

6. Deft Explorer – Roving (Ranger). 6th-level feature that grants a swim speed equal to your walking speed.

7. Gift of the Depths (Warlock). 5th-level Eldritch Invocation that grants swim speed.

8. Storm Soul: Sea (Storm Herald Barbarian). 6th-level feature that grants 30 feet of swimming speed.

9. Gift of the Sea (Fathomless Warlock). 1st-level feature that grants 40 feet of swimming speed.

10. Wild Shape (Druid). At 4th level, Druids can Wild Shape into creatures with a swimming speed.

Racial Features:

11. Water Genasi. Swimming speed is equal to walking speed.

12. Lizardfolk. Swimming speed is equal to walking speed.

13. Sea Elf. Swimming speed is equal to walking speed.

14. Triton. Swimming speed is equal to walking speed.

15. Locathah. Swimming speed is equal to walking speed.

16. Simic Hybrid. With the 1st-level Underwater Adaptation feature, Simic Hybrids can gain a swim speed to walking speed.

Magic Items:

17. Ring of Swimming. Grants a swimming speed of 40 feet.

18. Cloak of the Manta Ray. Grants a swimming speed of 60 feet.

19. Gloves of Swimming and Climbing. Grants a swimming speed equal to walking speed.

20. Ring of Water Elemental Command. Grants a swimming speed equal to walking speed.

21. Apparatus of the Crab. Technically a vehicle, but it does allow the party to have 30 feet of swim speed.