Why is my snake lying in its water bowl?

Pythons will often fully submerse themselves in their water bowls for long periods of time (even days), most likely to help soften the old skin during pre-slough. 

A brazlian rainbow boa (can't be kept as a pet in WA).

A healthy python will shed its whole skin in one piece and a couple of weeks before it sloughs, the new layer of skin underneath secretes a milky fluid containing an enzyme that softens the old layer, causing the new and old layers to separate.  A few days before the skin actually comes off, the milky eyes will clear and the python will become restless and start pushing its nose up against hard or rough objects to hook the old skin on its jaw and then crawl forward and out of its skin, leaving the skin inside out. 

Captive Stimson's python shedding.

During this time, you should include (clean) rough rocks and branches as part of the furnishings in your pet’s enclosure to aid in shedding.