Which proteins are most sensitive to electric fields?
There are various proteins sensitive to changes in electric field. More precisely, they are activated by changes in voltage (surrounding the protein), and immersed in water, as all cells are mainly composed of an aqueous solution. You are correct that most charges in the side chains of the proteins' amino acids are 'masked' by the dipole character of water molecules, but the electrochemical potential of the cell is primarily due to ions, and not to the interaction between side chains in a protein. However, it is important to note that within these proteins sensitive to changes in the electrochemical field, there are domains that contain charged aminoacids (i.e. not covered by water), very important to be able to 'sense' the changes in voltages. 

You can check out some of the voltage-sensitive ion channels, which participate in different cellular pathways, including the generation and transmission of the action potential in neurons:



Wikipedia has a good entry about them.