I HAVE A SMALL CAKE THAT LOOKS LIKE A POOL.. BUT I WANT A DIVING BOARD AND OR A SLIDE

Blue piping gel is great for water.  I put it on top of light blue butter cream that is wavy to give it a choppy look (great for lakes and rivers) or smooth bc for still waters of a swimming pool. 

For your slide, a strip of half fondant and half gum paste.  Make it as long as you want your slide.  Lay it on a dusting of powder sugar or corn starch so it doesn't stick while drying.  Make sure to turn ends under slightly like a real slide would work.  For your ladder, roll fondant over toothpicks or lollipop sticks, leaving part of the end exposed so you can poke into cake. For the ladder steps, thicker fondant cut into strips the same size, attach to you side rails and let dry. 

Once dry paint silver then after that is dry a light dusting of shimmering dust if you want. 

A diving board do the same way just keep it white.  Make a tiny spiral of rolled fondant to make it look like a spring and paint silver or use grey fondant.  With fondant, just pretend it is clay and come up with all kinds of swimming pool props.  An inner tube, a rafter, beach ball, umbrellas, even chairs. 
 

Make some gumpaste and let it dry until hard. You can "glue" it to the cake using royal icing.