The fragile-to-strong transition in supercooled water, where the relaxation dynamics shift from non-Arrhenius to Arrhenius behaviour, has been hypothesized to explain its anomalous dynamic properties. However, this transition remains unresolved, as previous ultrafast experimental studies of bulk water dynamics were limited to temperatures far from the proposed transition due to rapid crystallization. Here we use an infrared laser pump and an ultrashort X-ray probe to measure the structural relaxation in micrometre-sized water droplets, evaporatively cooled at timescales ranging from femtoseconds to nanoseconds. Our experimental data show a dynamic crossover at around 233 K. Below this temperature, the relaxation dynamics deviate from simple power-law fits and follow a shallower temperature dependence. Molecular dynamics simulations successfully reproduce our findings.