Your horse can't swim across the water. Use your agent to build paths to connect the islands so that you can ride your horse across.
You will need to build a path for your horse
The path should be 2 blocks wide to make it easier to keep the horse on it.
If you get stuck, get a lead from Dio to lead your horse back to dry land.
There are lots of different solutions to this puzzle.
Since there is no one solution to this puzzle, there is no specific hint.
One suggestion is to take your code in sections, one island at a time.
Think of this puzzle as similar to the Broken Bridge puzzle, and use similar coding strategies for each stretch between islands.
Your agent won't be able to place blocks outside of the area contained by the buoys.