The study presents an interaction system named SensorTank, which can detect the position and volume of objects such as human feet inserted into water. SensorTank is designed to be used as an interactive water vessel interface in which three-dimensional interactions occur. Feedback is provided by visual, auditory, and thermal sensations. For detection of inserted objects, combinations of phototransistors and red lasers that form sensing units are arranged on four sides of the vessel. Signals given from the phototransistors are interfaced via an Arduino Uno microcontroller and a multiplexer circuit. An elementary application has been implemented to illustrate the use of the tank for foot gestures, and early experimental results suggest that the proposed mechanism is both feasible and practical even under murky water conditions.