In the experimental 1.18 snapshot, swamps spawn in small patches. Usually surrounded by other water biomes. 

Below are examples

Swamps may spawn long and thin (Seed: -3063781826601937332 , Coordinates: 9178 ~ 9974)

Or in small patches in large bodies of water (Seed: 1683232707900911030 , Coordinates 1404 ~ 56)