You probably have taken a part of his, as having added the behavior of the platform to your character, so let's move:
How to make water in your platform game
Create a tiled background (with name "Water" or "Water" to make it easier) fully populated with the following colors in 32px x 32px size (recommended):
Then go to the event sheet and add an event:
Add event> (Your character)> Is overlaping another object> "Water" or "Water" (according to the name that you placed)> Add Action> System> Set time scale object> (Your character)> "0.5" > Add action> (Your character)> Set jump strength> 330 (preferably).
Add another event:
Add event> System> Else> Add Action> (Your character)> Set jump strength> 650
Copy the action: "Set object time scale" event in event created earlier and change the value 0.5 to 1.0.
Ready! But remember, this will make your character jump and lower it out of the water, depending on circunstanscias is better to put stairs, ramps, or use the "wall jump" (visit my tutorial on how to create it: https :/ / scirra.com/tutorials/975/como-fazer-o-personagem-dar-um-wall-jump)