Ma'aser Sheni 1:5 says that water is an invalid option for buying with ma'aser sheni money. The reason, as Bartenura's commentary explains, is because ma'aser sheni money can only be used to buy things that grow from the ground.
However, the sechach (roof) of a sukka also has to be made with things that grow from the ground. This is learned (according to one opinion) from the verse that describes a cloud of mist that rises from the earth (Genesis 2:6). Just as a cloud grows from the ground, so too a sukka has to be made with a substance that grows from the ground (Sukka 11b).
If water is considered a substance that grows from the ground for the purpose of building a sukka (even as the prototypical case), why is it invalid as an option to buy for ma'aser sheni for the very reason that it doesn't grow from the ground?