GERONIMO is a password that gives you infinite lives, as you can go head-first into everything in the game without fearing a game over.
This would appear to work, however there are no lives in Ecco the Dolphin so you always have infinite lives, even if you don't enter the code.
If you press Up Down Left Right A Start at the title screen, you can enter the level select.
This was printed in some game magazines, but the code was not intended for Ecco. It is actually the level select code for Sonic, however incorrect positioning lead some readers to attribute this code to Ecco.
By simply staying as low as possible and constantly moving through the opening 5 levels, you will decrease the damage enemies do to you.
It is very difficult for new players to constantly move along the bottom, and they would often die by attempting this.
If you swim through the rock borders at the beginning of the game, you can reach a level select screen.
Like most 16-bit games, collision detection is not perfect and it is possible to glitch through the level borders in certain areas. Of course, this is very difficult to do, and does not lead to a level select screen
HCBTLAEW is a password that would make fish restore all of Ecco's health.
Due to the fact that you can easily heal to full using fish, this might appear to work. It fits in the password screen, but does not actually do anything. However, it is a Gameshark code which does what is advertised.