Your parts guy is the Moron !
But he is only HALF correct. Yes, MOST greases are water "resistant" to some degree. MOST automotive greases are Lithiums. Lithium greases are considered to have "relatively good" water "resistance" and are correctly referred to as having good "water-washout-resistance" qualities, hence its wide use in the automotive industry. However, GP or General Purpose Automotive Lithium greases, unless specifically formulated are NOT considered to be 'waterproof' in the strictest definition.

The term "waterproof" grease is often used incorrectly. With grease it is called water "resistance" and there are ASTM test methods to determine this.

There are many different types of lithiums and each one has its own water resistant capabilities, without getting too technical, not to mention that each manufacture has their own formulations, which also affect the water resistance of a grease.

NOTE : *MOST "quality" auto greases will give suitable water-washout resistance for use as a cycle lube.* The caveat here is that, as in all things, some greases ARE better than others.

Typically, ""highly-water resistant"" greases are also referred to as Marine greases or waterproof grease and are often just highly formulated Lithiums anyways. Marine greases often use higher base oil viscosity's and polymers and or tackifiers. (I have a synthetic marine grease for my boat trailer that is tacky as bubble gum.) IMHO, these are too thick and gummy for bike bearing use.

Technically, the best "waterproof" greases are not Lithium, they are Calcium or Aluminium Complex and many others, the absolute best is Barium, but I think it's considered to be toxic now, so I won't recommend that you use it...... ;>)"