A weather-resistant body coupled with weather-resistant ("WR" designated) lens is fine for light rain and splashes. Just be sure to wipe the whole thing down occasionally, as standing water around joins and seals is where you're most likely to suffer ingress. Also, if it's salt water, it can corrode some metal parts (even painted ones) over time if not wiped off.
As for a roll-over, the best protection would be afforded by an underwater photography housing designed for your specific camera and lens. You might get away with just a water resistant body and an "AW" all-weather lens if submerged very, very briefly, but there's no guarantee. Weather resistance in cameras and lenses is really designed to reduce the chance of ingress from splashes, rain, dust etc. - and it's not guaranteed weather proofing - merely increased resistance
Note that if you use a weather-resistant body with a non-weather-resistant lens, all bets are off. That said, I've had very light rain on my old, non-weather-resistant *ist DL before, and with regular wiping to remove standing water, there have been no problems. YMMV, though.
I'd recommend you read the following thread by fellow member @stevebrot - there's lots of useful info from Steve and others:
https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/351252-about-th...-practice.html