Swimming in source ports is the ability to move vertically within deep water areas. It is generally implemented alongside flight in ports with this feature. However, contrarily to flight, it is a permanent ability; the player is generally able to swim anytime their character is immersed in deep water, without having to acquire a special powerup first.
Additionally, some ports may assist the player in trying to swim out of a pool of water, by "climbing" along a wall or other obstacle when pushing forward against it, and by jumping automatically upon reaching the surface.
Note that Boom line type 242 does not feature the ability to swim, despite introducing deep water. It is often used for non-water effects. For this reason, Boom-compatible ports do not allow swimming in Boom-style deep water. Creating swimmable deep water requires either an explicit swimmable water line type such as 280, 301, or 304 in Legacy, or an additional step such as placing a "water zone" actor in ZDoom.