Water has quite low thermal conductivity (~0.6 W/(m*K)). If you exclude convection you can easily have such a temperature difference. For example with heat flux

Water has quite low thermal conductivity (~0.6 W/(m*K)). If you exclude convection you can easily have such a temperature difference. For example with heat flux of 100 W/m2 you have 60 K temperature difference over a 36 cm thick layer of water.