Almost all the non ladder maps i play have underwater mexes. Flooded tabula, flooded argon, flooded corona, fields of the great phoenix, point of reach, diversity, twin rivers, neptune, flooded monument valley, flooded funeral plains, anchor, africa, etc. Newer water maps like Nihau island or Chiron also have underwater mexes.
There are basically only 2 water maps that are played a lot and don't have underwater mexes. Setons and dual gap. There are a ton of water maps without any underwater mexes but if nobody ever plays them it doesn't matter anyway.