Minecraft wiki entry for Building_water_features, associated text: Make the bottom of the floor concrete. Polished Diorite, Block of Quartz or Smooth Stone are some alternate choices.
Build a 3 wide path around the perimeter using concrete or other blocks. Dark Prismarine and other prismarine blocks can be a great accent material.
Make diving boards out of quartz, quartz slabs, and ladders. You can make them 1, 2, and 4 blocks high for low, medium, and high dives.
Put potted flowers in each of the corners of the pool.
Use wood planks and ladders to make the base of a lifeguard chair. For the chair, use a wood stair with item frames beside it. If you want a roof to the lifeguard chair, use lime wool, using fence posts to connect the chair to the roof.
Use quartz slabs and light blue wool to make lounge chairs.
Plant saplings around the perimeter of the pool, and grow them into trees. Make sure you place them far apart from each other and not too close to the pool, because trees can grow very big sometimes.
Use glowstone or sea lanterns for underwater lights (not shown in picture).
Add a smaller, shallower pool nearby, for a hot tub or kiddie pool (not shown).