Subs avoid terrain using charts and dead reckoning, including inertial navigation systems. They typically come to periscope depth occasionally to get a navigational fix. In 2005 a US submarine allided with an underwater mountain due to a navigational error.

https://www.cbsnews.com/news/whos-to-blame-for-sub-accident/

There is little or no light at typical operating depths. Lidar can work to a limited extent in some water conditions but it's not something to rely upon for collision avoidance.

I remember (barely) from a diver friend that colors disappear below 20 meters (everything appears blue, unless a bright light is shone), and sun light disappears completely below 70 meters. You'd assume that an optical system with flood lights could theoretically work, but then it would be pretty simple to monitor for light at these depths (for counter-intelligence/retaliatory purposes)