A marine substation for a grid-connected wave power plant has been designed and constructed at Uppsala University. The measurement system for the substation has been developed from basics, and the procedure is described in this article. Subsea electrical installations set high constraints on the control and data acquisition systems used, and traditional GSM networks and GPS time synchronization are difficult and inefficient. These circumstances exclude many well-proven methods to be utilized. A compactRIO-based system, with integrated real-time controller and Field-Programmable Gate Array chip, is chosen as most suitable for this task. The system is designed to meet the special requirements encountered in this unusual application, including large data sampling, grid connection control and protection systems. The system is communicating to shore via a subsea copper cable and single-pair high-speed digital subscriber line modems. The design was successful and full-fills all the system requirements. The aim of the article is to provide future researchers with a good design and implementation procedure for setting up large measurement systems.