In this paper we propose WELOC, an ultrasoundbased localization system for localizing objects in a marina environment. WELOC uses the already installed infrastructure of commercial battery-powered IoT devices, with several years of autonomy, fixed at each slip on the pontoons, used for boat presence monitoring. These devices are equipped with an ultrasound sensor, have known GPS positions and form a wireless, time-synchronized (<15 us synchronization error), mesh network around the gateway. Without impacting the main function of the smart parking system for boats, we propose a scheme to schedule when each ultrasound sensor is triggered. We design a mobile device, from an off-the-shelf ultrasound sensor, compatible with those already present in marinas. We synchronize the device with each ultrasound trigger in the marina, and collect the timestamped distance measurements. The location of the mobile device is then computed using trilateration. We test the ranging accuracy of the mobile device in the lab, where we achieve a 3.5 cm mean absolute error, with a maximum range of 10 m. We perform field testing for the ultrasound signal detection inside the marina, which is equipped with 471 presence sensors, one at each of its slips. We show that our mobile device successfully detects the ultrasound signal on the pontoons.