The idea is brilliant! But the implementation is rather bad.
There's always a sudden change of perspective. In first person mode, my mouse was way too sensitive and there's no way to adjust that. In side scrolling mode, it took some time until I figured out the controls have changed and down actually meant descending, not going backwards. It would've been better to stick to just one.
The submarine goes way too slow. My finger hurt after I had to press a single button for a good half an hour. Also almost nothing has happened in that interval of time, not counting discovering some artifacts that I didn't even notice, and didn't understand why the big letters showed up, so a camera focus on them would've been nice. But speed is a bigger issue. I understand the slow paced exploration stuff, it sounds nice and all, but a submarine simulator in empty water gets boring after a few minutes. Yes, it's deep water, there's hardly anything there, but it's not real life, it's a game and it needs to be entertaining. Fill the space with events, visuals, or audio at least.
There's no voice after some point and I thought there was something wrong. There's not enough time to read the conversations after the voice is off.
And I never knew when I'd die, I kept bumping into walls, never knew where to go. Sometimes you need to proceed by going through a place that looks like a wall which may or may not actually be a wall. It's a 50/50 chance of dying or reacing a new area. The radar didn't help either.
So I died and the last save file was way too early and I didn't feel like trying again. An auto save at the discovered aftifacts could've helped to feel more like trying.
The whole experience was sheer suffer, I wish it was as great at it looks like on the pictures. Maybe one day... :)