There are really a lot of option so it will come down to what provides the particular look you are wanting to create. With a pool it is often a good idea to choose a plant with bold blooms and/or foliage so as to create a tropical look.
Some plants to consider include:
Hardy Cannas (such as Tropicana, Richard Wallace, Black Knight and Futurity Pink), Daylilies, 'Midnight Blue' Agapanthus, Hardy Fiber Banana (Musa basjoo)(tall so put in corner to not obstruct view), 'Harbor Belle' Nandina, Hardy Ice Plant (Delosperma cooperi), Purple Heart (Setcreasea pallida) and 'Miss Huff' Lantana.
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