Yes, it rarely crashes. However, we do not suggest doing that. Waterspeed, during navigation, uses a lot of resources of your Apple Watch, in order to track very precisely. Using other apps (then sharing GPS, Workout and resources) can affect performances, and sometimes, even if rarely, it can crash Waterspeed and the other apps. It is basically a limitation of the hardware of the device. 

Furthermore, another app could "steal" the workout mode from Waterspeed, making WS not run properly in the background and not able to read HR, calories, etc.