Deep Descent was too ambitious for its budget. A team that really loved Archimedean Dynasty/Schleichfahrt and the Aquanox games wanted to create both a prequel to the series and a potential starter for a new series. When it was to be released, and Kickstarter backers got a pre-release version to try, they noted that the game still felt very unfinished and unpolished. As a reaction, the development team strained their resources and held back the release for almost a year, working hard to fix the most glaring issues.
They did not fully succeed. The game does not hide its weaknesses. Storytelling is not fully voiced. Some graphics are missing. The ship customization is neither as detailed nor as well balanced as it was originally intended. Difficulty jumps up and down quite a bit during campaign progression. And moving through the semi open world is a bit of a chore.
Still, you can feel the passion that went into creating this game. The storytelling may be on the predictable side, but the story is well written and told. Gameplay is solid and enjoyable. The underwater world is beautifully crafted.
This game would have needed a proper budget and it could have easily been a surprise hit. The way it was released, it stayed a niche product. If it was to be added to GoG in a preserved version, the most glaring issues would be the UI menus, dialog sequences and transition between stations and game world that need fixing. If I recall correctly, those did the biggest job in breaking immersion within this game.