2 block Cactus (or higher) dont broke with water,but 1 block cactus can.
In the next images we can see the diferences between 1 and 2 block cactus.
1) Cactus of 1 block breaks with water
2) Cactus of 2 blocks dont broke
If this bug its fixed, the comunity can desing new and better cactus farms.