There isn’t really any way I know of to achieve wireless ROV control underwater for a reasonable size and cost. Using a tether is a much better solution in terms of cost, bandwidth and range. Why does your ROV need to be wireless? And how deep do you intend to dive?
Radio modules won’t penetrate water to a significant depth unless you’re using a extremely low frequency (3-300Hz), which would in turn require a very long antenna (up to 60km for communication with submarines). VLF (3-30kHz) will only work up to about 20m depth. The data rates are extremely low at these frequencies. Higher frequencies will only travel a few cm in water.
For greater depth and range a better option than radio is to use an acoustic modem, but they are usually over $2000 each. You would need one on the ROV and one on the surface for control. The data rates are much too low to stream video (100bps - 9000bps).
Another option is to use a tether from the ROV to a surface buoy and radio from the buoy to your base station. But this will be affected by surface winds and conditions, if there is any surface spray it could drastically reduce the wireless range. It will also effect the handling of your ROV. But it might be a good compromise for what you’re trying to do, if using a tether to your base station isn’t an option.