With crepe myrtles, you not have to water constantly but you have to water deep. That means, give them a good soak every five days or so and make sure that they are not just wet on the surface by checking the soil around the tree with a trowel. Once they are established, you only have to water them in extreme drought. I give mine a tablespoon of epsom salt every 3 months for fertilizer while getting established and now, I only fertilize them once a year. My neighbor does not do a thing with his and they look great.
Crepe myrtles are very hardy and tough so I know yours will bounce back.