Simulating Antenna in water using CST

Hello friends,
I would like ask a question regarding simulation in CST. I'm designing an antenna to detect the presence of E.Coli bacteria inside the water. At the moment, I managed to come up with the design and simulate it without any difficulty in CST. However, when I introduce the model of water material in the CST by inserting the whole antenna inside a brick of water, after running the simulation the results of the antenna performance deteriorates significantly. This is to be expected I guess so I would like to ask that are there any form of solution regarding this matter ? Do I need to add any other specific material for the antenna design to enable it to be simulated under water ?
Any help would be greatly appreciated. Thanks.. :)

I presume you know that an antenna design is based on a specific permittivity of surrounding medium (e.g. 1.0 for vacuum/air) . It has to be redesigned for different media.
Apart from this general point, water has a rather, high frequency dependant loss factor.