The Swift Turbidity Marker is an optical instrument developed to measure the level of water turbidity. The components and configuration selected for the system are based on common turbidity meter design concepts but use a simplified methodology to produce rapid turbidity measurements. This work is aimed at high school physics students and is the outcome of simplifying university research into a school-level project, while maintaining important standards dictated by international organizations such as Environmental Protection Agency (EPA) and International Organization for Standardization (ISO). The Swift Turbidity Marker is designed using a basic optical instrumentation principle that consists of an light emitting diode (LED) and a photodiode (sensor), together with supporting circuitry for data processing and presentation. This work integrates the physics of optics and electronics with simple mathematical applications for data analysis. (Contains 11 figures and 1 table.)