The first issue with being submerged in water is the additional propagation loss in the water: Water being conductive severely attenuates radio frequencies. So it would be key to stay at a low depth. Better link budget, ie. directional antennas and higher transmission powers, can help overcome this issue to some extend. The second issue is refraction between the air-water mediums. Refraction happens when a wave travels through a medium of one density into another medium of a different density. This causes the path of the wave to bend and also slows down the speed of the wave which in turn decreases the wavelength, which an underwater receiver must also account for. In short, the underwater scenario is very challenging and it would be advantageous to communicate at the surface.