One is to reach certain temperature, the other is to maintain the body temperature.
Explanation:
Since both are to keep warm some reactions of substances, the water bath can have temperatures since 25 ºC until 70 or 80 ºC, in the case of the incubator, the main function of this aparatus is to simulate the body temperature of a living thing. Us as humans we need a body temperature of 36 - 37ºC.
