help with entity spawning blocks overwater when stepped on

On the mobs tick update in the triggers tab, make it check if the block below it is water, then replace with cobble stone. 

hey GranKnight could you post a image of how you would do this please

In order to do what Granknight is saying here, you would first create a new procedure and select the global trigger of "On entity tick update." From there, you would do an if then statement of 
if [is (get block at (x) (y-1) (z)) the same block as (minecraft:water) then]
[Replace block at (x) (y-1) (z) with (minecraft:cobblestone)]
Here's an image to better explain:

thank you bro i got it working

Have the trigger be entity tick, then check if the entity is a subtype of [mob you want] you can find it either by searching for it or in logic tab, and have that before the checking if block below is water.