The TerraFirmaCraft water variants tfc:river_water and tfc:salt_water do not waterlog blocks such as fences, slabs, stairs, etc.

River water is functionally fresh (vanilla) water and reports itself as such. Stairs, slabs, fences etc. are all coded with vanilla waterlogging in mind (i.e. they don't accept lava either) and so don't accept any modded fluids, including salt water.