There's no indication on the watch itself when it's fully charged, and the battery indicator only appears when the watch has like ~4 hours of battery remaining. Thus it's hard to get a read on just how long the watch needs to charge for -- I usually plug it in overnight once I see the low battery indicator, then wear it for another week or so before the low battery icon shows up again.

As for things to do with the Pebble during the shower: use it to control your music!