Locate the difficulty files in your Crysis installation folder (ie: C:Program FilesElectronic ArtsCrytekCrysisGameConfig). These files are called diff_easy.cfg, diff_normal.cfg, diff_hard.cfg and diff_bauer.cfg. Open the file (with Notepad) that corresponds to the difficulty level you are playing. Add the following line to the end of the file and save it:
g_godMode = 1

Now, when playing your health will still decrease but you will not die. Instead, your health and armor will automatically replenish when your health reaches 0.

Unlimited Ammo
Using the same method as above, add the following line to the end of your config file:

i_unlimitedammo = 1

Your ammo will be frozen in the game.

NOTE: ALWAYS MAKE A BACKUP BEFORE OVERWRITING GAME FILES.