In the field of deformation monitoring and damage assessment, there have been few approaches to high-precision underwater methods so far. Due to their limited resolution in the centimeter range, acoustic sensors are suitable only for large-scale damage detection, whereas optical measurement methods promise higher resolution and greater detail. The Underwater LiDAR System (ULi) offers an active optical measurement method in the form of an underwater laser scanner for 3D mapping of underwater structures. Previous studies show measurement precision in the millimeter scale and maximum measurement distances ranging from one to two times the visibility range in water. With up to 100 000 measurement points per second, ULi is comparable to terrestrial laser scanners and offers the potential for high-resolution condition and deformation monitoring of, for example, underwater constructions. However, similar to terrestrial laser scanners, ULi measurements are influenced by numerous factors related to the equipment and the environment. For example, effects related to ambient temperature can be observed in the millimeter range, and warm-up effects can take several minutes until the laser stabilizes. To investigate temperature-dependent measurement characteristics of the sensor system, measurements were conducted in a controlled laboratory environment with varying water temperatures. In addition, the laser’s measurement behavior during 30-minute warm-up phases is analyzed and described through successive scans. This paper provides an initial quantification of thermal influences and confirms that temperature must be taken into account in high-precision underwater laser scanning applications.