Given the choice, I will definitely not transmit 2GHz under water. This is a situiation we have to deal with whether we like it or not. I just found out deionized water loss tangent is 0.048, it is much better than 0.17. This helps a lot.
demineralised and deionized water often used as interchangeable descriptions. Even the purest water contains H+ and -OH ions in equilibrium check your handbook
you may be refering to water with dissolved species, such as saltwater. the loss tangent for various solutions at various concentrations is what you need to look up