You need to determine whether the bath of water that the box is submerged in can be thought of as an infinite heat sink  so it maintains a constant temperature  or not. If you can assume it's infinite then you can treat it as a heat sink just like you would treat the air, so you use your formula twice  first with area of the box exposed to the air and then second adding the effects of the area of the box exposed to the water. It also appears that you are treating the box as having a uniform temperature, which makes the math relatively easy, but is that a reasonable assumption? Or  do you need to determine the gradient of temperature inside the box?
