Change controller gravity below a certain level?

Hi there im trying to create a code for a controller that will casue a drop in gravity when it enters water, to even make it float to the top once it falls or something along those lines, i have worked with creating the water level since ive managed to add camera effects to the camera to create fog for the water and even a bubble effect on the particle system attached to the controller but i have no idea where to start with the controller itself, ive tried it with the envrioment's grvity but no use this doesnt effect the controller.