Nice it cas an immersive experience! I found the player a bit slow too once inside the submarine and the intro is quite long as you cannot speed-up the dialogs. That beiing said it was well polished with a good atmosphere and it forces you to take the pace of the deep ocean. Lights effects were cool.
I would have find more immersive to add some flora and fauna even if it's juste shady shapes moving in the background somewhere... during the descent.