Good advice by Longbow. Keep at it is the best advice. It's very difficult to say more without spoilers. The Mk 2 seamoth depth upgrade is good, but there is far deeper sea to explore. You'll need to continue to upgrade it (or get the Cyclops/Prawn). I agree with Longbow that the Cyclops isn't needed, but you will want one of you build a Prawn.
I'll put this in spoilers, and I encourage you not to look. These sorts of games are better if you find your own way, and solve things, without hints in my view. But if you are truly stuck maybe it will get you on your way.
Well, I kept the spoilers brief. Just a nudge. Of course we can give more detail if needed, but better to start off on the safe side. You need to continue to develop gear that will allow you to dive deeper. I mentioned in the thread I linked to in the second post that as much as I like the game, I eventually grew weary of continually needing to chase the upgrades that would allow me to go ever deeper and pursue the storyline.
"I have grown a little weary of upgrading my vehicles. I need to do so to go ever deeper and handle the flora, fauna and hdyrospheres that the player encounters as he pursues the storyline. Crafting isn't difficult in Subnautica, but as you know you'll need the materials and with the extensive options it can be time consuming to track it all down and get it all to your base to build the stuff. No one is forcing my pace or choices, but the sense of wonder and discovery is a little intoxicating and to pursue it I need to build more and better equipment.
It's not that bad, especially compared to other games I have played. But depth capability is key to making progress. The fact that your Seamoth can now dive deeper is good. It opens a whole new strata of the game world. Use that new capability to go places you could not before. This in turn may lead to new discoveries and blueprints that will lead to others down the line.