Before any dive with a camera there is one thing you can do that will prevent a vast range of grief, embarrassment and anguish. The test shot.
It doesn't have to be anything special. Mine are often pictures of tables in the saloons on dive boats like the one below. But I also do bits of camera kit, selfies or unflattering shots of my better half.
What the test shot does is show that you can get a picture from your setup before you get in the water. If you can't get it to produce a picture or it looks wrong you can fix the issue in the dry. Some problems can be fixed underwater others can't. A camera not being aligned properly in the housing will lead to a frustrating dive where none of the controls work and you can't fix this without going back to the boat to open it up.
The test shot can detect all sorts of problems from a strobe cable not being plugged in or even left in the back of the car on a shore dive, to a compact camera being put in upside down in the housing (this can be done!). Other classics include leaving the lens cap on the camera, not putting batteries back in the camera, putting the flat battery back in instead of the charged one the list goes on.
Taking test shots also allows you to get things somewhat setup for when you are in the water. You can get an initial handle on positioning of strobes and/or your camera settings by taking a string of images before you get in the water.
The picture below is of well known British underwater photographer Nick More's finger on the breakfast table at Tasik Ria Dive Resort in Indonesia. I took it using a snoot before the first dive I had ever used one on. Practicing before hand gave me an idea on how to use it and I got some pretty good shots my first time out with what can be a frustrating piece of underwater photography kit to use.