Clear a corner of the pool of any debris and other items that may get in the way. Build a rectangle out of 2-by-4 boards that reach across this corner of the pool. Make the rectangle as wide as you want. Cover the rectangle with latticework using staples and an electric stapler.
Nail 2-by-4 boards into each corner for legs that are as tall as you want the structure to stand above the surface of the pool. Paint the structure with waterproof paint. Place each leg of the structure in a decorative planter and fill around the legs with heavy gravel. Add a couple of inches of white rock on top of the planters.
Move the structure into place, spanning the corner. Staple more latticework on the sides of the structure. Fill two long rectangle planters with soil and plant fast growing vines. Move these planters under the sides of the structure. Train the vines to grow up and over the top of the structure.
Fill more decorative planters with soil, and plant spreading bushes. Use a mixture of evergreen plants and tropical plants with large flowers. Spread these plants around the outside of the structure. Plant taller varieties of plants in large urnlike planters to fill in the back of the structure. Leave an entryway in the back so people can enter or exit the pool that way.
Plant a few large ferns in heavy pots to set beside the water inside the structure. Hang pleasant-sounding wind chimes and hanging plants in lightweight planters from the top of the structure. Place a couple of fountains behind the ferns so the sound of running water fills the grotto.