Where is the water pump under the hood of a jeep wrangler?

This depends on which engine your wrangler has in it as to exact location and type.
On the 2.5L 4-cyl, 4.0L & 4.2L 6-cyl it is on the front center of the engine behind and attached to the pulley directly above the bottom pulley (Cranshaft pulley).
On the 2.4L 4-cyl it is above the bottom center pulley (Crankshaft pulley) and a little to the driver side front of the engine.

As a general rule, find your "bottom" radiator hose and follow it to the engine. In most cases it attaches directly to the water pump.