Transform Item When Thrown In Water

Hey,

I would like to create a procedure, which can transform an item into an other item, when I throw it into a 1 block water hole.

My idea:
- Using the "Gem Dropped" Global Trigger
- When a player drops a specific item ( for example Item A) in an 1 block water hole,
  there are many conditions for checking, if there is a solid block (or rather: no water block) around the 1 block water hole,
  where the Item A is thrown in.
- If all the conditions are true, Item A gets consumed through a World Command Execution: kill @e[type=item,r=2] at x y z
- After that a new item (Item B) spawns at the same place in the water at x y z

Problem:
- The Global Event "Gem Dropped" gets triggered, when the item drops, not when the item touches the water (that makes everything difficult)

Possible Solutions:
- Create a delay between the procedure blocks, to pause the Procedure until the Item A is thrown in the water hole
But i don't no, if that kind of delay is even possible...
I looked up for such a solution but Block Delays, Global Timers and such things didn't worked for that.

Soo... hopefully anyone can understand my problem and maybe help me solve that ;)