Can't confirm that the size of the water makes a difference. I've had different sized dishes in use, from fairly small, covering about 1/6 of the surface to dishes covering 1/2. The male shows absolutely no interest in water and crosses it as quickly as possible. The female rarely spends more than 30 seconds in it at a time. Both have occasionally tried to find a way out of the terrarium through the bottom of the dish.
Of course, I have no fish present and neither snake shows any interest in live fish anyway.