Whether something is 'explosion proof' has no connection with whether it is weather proof, oil tight, or submersible. These are separate ratings. There is no "hierarchy' in the NEMA classifications- it's best to consider each as distinct.
Since one of the more common engineering solutions to making something 'explosion proof' is controlled venting, any form of gasketing or sealing could very well defeat the venting.
That said, appliances DO exist that are both submersible and explosion proof. I am thinking, in particular, of some rated sewage pumps. Keep in mind that water can carry with it other fluids, and many of these fluids can separate and become vapor as they move through a pump. Likewise, the pump body can become exposed should the pump run longer than it should.