Mud

You can create a bucket of mud by crafting 8 dirt around a single bucket of water. Mud flows like lava and will slow down the player whilst swimming in it. Pigs become muddy pigs when in contact and it can have infinite source blocks. It can even be put in the Nether, but only the source block can exist. If anything tries to spread, the spread will dry up. It won't completely stop you're fall, like lava, but it can be used to put a player out if set on fire. It also dries up in contact with lava. When mud dries up, it becomes a brown version of clay.