We didn't get it explained very well tbf. We had more we were wanting to do and not enough time to implement it. 

While descending you have free movement to swim as you retrieve things. Both the blue air tanks and the planks when you pick them up switch you to a mode of platforming as you return to the ship. You'll want to pick up the tanks since they should be extending your dive time. There's also  a bug with movement that gives you massive acceleration that we weren't able to fix because it wasn't very consistent and needed to upload. 

It might be an issue with the build now that I've seen someone try to play it. Was your movement speed super slow?

Edit: yeah it seems the build we uploaded has a major issue with the movement that's probably why people are struggling. t.t

Edit 2: got permission to re-upload a version fixing the movement. It should be working now and probably will make more sense to play. Lemme know if you have any other feedback! :D