The Fairphone 2 was designed to be a robust device, but it is not ruggedized or waterproof. We wanted our new phone to be easy for anyone to open and repair, which led us to create a unique modular architecture. We took great care to keep water and dust ingress (a common cause of failures) to a minimum by employing careful construction and seals and gaskets in the necessary places. This should enable the device to withstand normal usage conditions, but you should take care of your phone nevertheless.
Moreover, making a completely waterproof smartphone would require a more sealed architecture, which would have been far more challenging considering our ambitions for openness and repairability.