No, OpenSprinkler does not have USB host functionality, so you can’t use a USB adapter.
Please check this section of the user manual for recommended WiFi Adapters:
http://rayshobby.net/?page_id=3775#network
Also, this blog post explains how to power the WiFi adapter through the USB port:
http://rayshobby.net/?p=4233
I would say it’s ok to put everything in a water tight box. I haven’t tried this myself, so I don’t know for sure. But the controller consumes less than 1 watt of power during operation, so I assume it shouldn’t produce much heat at all.