Hi Mikxie,
We focus the camera in air to about 50cm or so, we find that this gives the ideal focus in water at the distance we normally observe. We find that there is a specific “sweet spot” which gives good focus close up and further away, we focus the cameras to this by default. Generally, there is no right or wrong focus- it just depends on your personal preference and how closely you’re going to be looking at things.
-Adam