Waterproof ultrasonic distance sensor. This sensor can be used for collision detection in your project.
Ultrasonic Module HC-SR04 Distance Measuring Transducer Sensor.
This is the SRF05 Ultrasonic ranger made by Devantech. The Ultrasonic detector can detect from ~4 inches to ~13 feet.
RPLIDAR A2 LIDAR is a 360-degree two-dimensional laser range scanner (LIDAR)
AMR (Autonomous Mobile Robot) for patrols and security. Options for object detection, automated pathing, indoor/outdoor functionality are available.