I'm doing a project that requires a platform that is able to float up and down a water depth of 0-100m. It'll be pulled down by a fishing line. I'm currently using air as buoyant. This air will be trapped in a container. However there is air pressure at a certain water depth. The air will certainly be compressed at a 100m water depth. What volume of the air should i use? Is it the volume of air needed to float the platform at atmospheric pressure(sea level) or at the 100 water depth. If i were to use the volume of air to float the platform at 100m, wouldn't the net upward force(buoyancy force against the weight of object) acting on the platform be greater?