swimUpVector is not a variable you make in construct its a number you set based on how high you want the jump to be, if you want the player to go higher set it to like 500, that will be a normal jump (i think 650 is default jump height in construct 2 platform behavior.)
so it should look like
add event: Keyboard>KeyIsPressed>Space (or whatever you use for jump)
add condition: Player>IsOverlappingOtherObject>Water
add action: Player>Platform.VectorY = Player.Platform.VectorY + 250