There are primarily two ways a water pump can fail: the bearing wears
out or the seal leaks. Usually the seal goes first and cause corrosion
to set in, taking out the bearing. Improper belt loads on the pump
shaft (bearing) is rare unless the mechanic is not competent, but this
will shorten the life of the pump as well. Wrong coolant, tap water or
low concentration are not good for your cooling system.

Where is the coolant leak? Under the timing belt valve cover? The pump
can leak from the weep hole or at the mounting gasket. Assuming your
cooling system is properly filled (no air in the radiator). Does the
coolant level in the reservior drop?

Take it back to the original installer of the pump is probably the best
bet, especially if you frequent that mechanic. After 1 year, even
Toyota will tell you warranty is over ($$$$$).

These import cars need silicate-free coolant. So use Toyota red coolant
($9.99 with coupon) with 50% distilled water only. Now everybody sells
only prediluted coolant, and if that's the case (Toyota pink) don't add
any water.

Airtex has a video on the diagnosis of cooling system failure. Check it
out:

 http://www.airtexproducts.com/prodlisting.html#