Of the sunken lifepods, Lifepod 3 is at the shallowest location, just 18m deep. Notable rewards for visiting it are the Compass blueprint and the Seaglide and Mobile Vehicle Bay fragments scattered around the area.
Once you receive the transmission from Lifepod 3 over your Radio, your Beacon Manager will contain its coordinates.
Lifepod 3 is located in a Kelp Forest biome. A Seaglide fragment sits on the ground near it.
Inside, along with the PDA containing the Crew Log, is the databox for the Compass.
The Crew Log hints at the story behind the Seaglide fragment outside the lifepod.