Undead do not need to breathe, and the reason why you can “drown” is mostly because of game mechanics.
Remember that the game is not necessarily reflective of the lore.
For instance, Forsaken were originally under the ‘Undead’ creature category, but this was changed to the more normal ‘humanoid’ creature category due to gameplay and balance reasons as certain spells, abilities and effects worked differently on different categories of creatures, I.E. Dragonkin, Humanoids, Undead, Elementals. For instance, paladins were OP vs Forsaken due to many of their spells becoming more effective when facing undead targets.
As far as eating is concerned; undead are animated by the unholy necrotic magic binding their forms into a verisimilitude of life. And even if they do eat, most undead lack a functioning digestion system.
Granted, the cannibalistic bloodlust of ghouls and zombies does suggest that the dark magic binding them gives them a certain level of rush when they ‘eat’ things, and there are some exceptions like Glut that have the ability to infuse themselves with dead matter.