https://www.dw.com/en/will-climate-change-cut-off-the-panama... discusses some possible solutions:

> solutions [...] include water-saving sluices that would collect freshwater in basins so it can be reused. To this end, possibilities are being examined to develop and exploit other water sources near the canal. The construction of reservoirs and saltwater desalination plants are also being considered.

There's also:

> The transport route from Asia through the Panama Canal to the US' east coast can be partially rerouted through the Suez Canal

whoops.