This game improves the gameplay, the enemies, the writing, and the graphics from the first game; but the level design is just BAD. I got stuck for hours and I needed to resort to a guide for 10/28 levels vs the last game's 4/26. There is also a certain atmosphere that's missing in this game vs the original that I miss.
The main new feature this game has versus the first is water "physics." You can very primitively swim, and instead of raising platform puzzles, this game consists of water puzzles. It's really annoying to control and I really wasn't a fan. You can't use any guns underwater except for the fusion pistol, which also hurts you upon use, so you end up having tons of underwater fistfights that take forever.
Overall, I do actually believe this is a worthy sequel. Just with some glaring issues that were too much for me. I don't regret playing it; the lore greatly interested me, and I would still recommend to anyone who enjoyed the first.