Steel Diver

Chapter 6
Successfully complete Chapters 1 through 5 with all three submarines to unlock Chapter 6.
Chapter 7
Successfully complete Chapter 6 with all three submarines to unlock Chapter 7.
Expert mode
Successfully complete each chapter with all three submarines to unlock the Expert difficulty.
Decals
Collect the following decals to unlock the corresponding effect. They are listed in order from left to right and top to bottom.
1. No decal: Default.
2. Anchor: Halves damage taken from collisions with the environment. (3 required)
3. Octopus: Halves damage taken from enemy torpedoes. (3 required)
4. Skull and Crossbones: Halves damage taken from enemy depth charges. (3 required)
5. Arrow through Heart: Halves damage taken from collisions with enemy vessels. (5 required)
6. Swords and Shield: Halves damage taken from mines. (5 required)
7. Wooden Ship: Quadruples recovery speed at the water's surface. (5 required)
8. Bomber: Halves damage taken from enemy air-to-sea bombs. (3 required)
9. Snail: Reduces damage taken from all sources. (7 required)
10. Orca: Halves air consumption rate of the masker. (3 required)
11. Lobster: Prevents submarine hull leaks. (5 required)
12. Bull: Increases torpedo damage. (7 required)
13. Golden Falcon: Increases maximum forward/reverse speed. (10 required)
14. Sea Serpent: Increases maximum dive/surface speed. (7 required)
15. Eye: Reveals enemy positions on the sea chart. (5 required)
16. Tridents: Increases torpedo reload speed. (7 required)
17. Lightning: Increase torpedo travel speed. (5 required)
18. Crescent Moon: Extends the mission time limit by 60 seconds. (5 required)
19. Scope Orb: Extends post-mission Periscope Strike by 10 seconds. (7 required)
20. Cherries: Extends post-mission Periscope Strike by 20 seconds. (10 required)
21. Sun: Increases torpedo speed in post-mission Periscope Strike. (10 required)
22. Panther: Decreases enemy speed in post-mission Periscope Strike. (10 required)
23. Crown: Lowers the health of enemy bosses. (10 required)
24. Scorpion: Negates the effect of water currents. (5 required)
25. Squid: Increases the effect of waters currents. (3 required)
26. Sea Horse: Halves the effect of water currents. (3 required)
27. Penguin: Halves the impact of shock waves. (7 required)
28. Wave: Grants a one-time health boost when reduced to zero health. (10 required)
29. Frog: Increases the impact of shock waves (3 required)
30. Fire: Increase the amount of bubbles created by your torpedoes. (3 required)
31. Mario: Homing torpedoes. (10 required)
32. Whale: Become a whale. (10 required)