The spawning floor to be between 24 and 32 blocks from you, so that mobs do spawn, but keep moving (and falling into the water).
Check your difficulty. No mobs spawning at all can only mean "peaceful".
The advice to seal all caverns within 128 radius is technically correct but practically not very viable. The best method to have no mob-spawnable ground removed anywhere but at the spawner is to build stairs 128 blocks above sea surface (sea, so that you don't die when you fall. You will fall a lot.)
This design of the spawner will not be efficient anymore. Monsters will avoid heavily damaging falls if they only can. It can be modified to work though: monsters can't plan ahead. Create a 2 or 3 blocks high, safe fall into running water, funnel the monsters to 1-wide corridor (all with water), and only then end it with a deadly drop. They don't foresee they won't be able to return and as they crowd in the 1-wide corridor, they will push each other to fall eventually.
Build your own, and have pt. 1 in mind.
Just to give you some hints, here are my designs...
Derelict of "Box", a failed design similar to yours. 1-wide water channels with 1-wide water drops simply don't work. Monsters won't sink in a 1-wide hole. And besides it was built far too low. The efficiency was abysmal. I dismantled it for materials.
"Circus", a very efficient design when it comes to monster spawning. It's a hemisphere about 25 blocks radius with center above the "house", covered with a roof and with water channels funelling mobs to a drop trap from 8 gaps around the chamber. It would be a great design if it weren't made of soil. That way it requires constant maintenance as endermen make holes in the walls and the roof, letting light in and ruining the efficiency. Never make anything important out of soil. Cobblestone is better but not optimal too. Use Slabs. And plan for maintaining the insides. Circus is only maintainable by switching difficulty to Peaceful. Otherwise, the inside is a deadly trap, and a creeper blowing up the wall - a disaster.
Why is it so patchy? Well, an endermen makes a hole, a creeper spots me and makes it much bigger, and the I seal the hole with whatever I have. Definitely, make your design out of enderproof material like slabs.
This is the Seaship/Mall.
It's entirely made of slabs, making it Endermen-proof, and it was an excellent choice. I botched the spawning platforms layout, and not nearly as much surface is located 24-32 blocks away from the player, which makes it perform worse than Circus spawn-wise. But still, there are entrances that allow for monitoring the inside, a cut-off piston that stops monsters from going to the dead-drop (which makes expanding the design easy), a spider-harvesting attachment - spiders tend to clog the walkways if left unchecked, a music disk farm, and of course the XP farm.