what are the circumstances which will cause a monocylinder byk engine's xhaustvalve get bent?

Answer 1

It's much more likely that the cam timing chain or gears have slipped a link or broken. If the engine had ingested water the damage would be done during the compression stroke when both valves are closed, usually resulting in a bent connection rod. I'm assuming that the bike is not running now, carefully check the cam and valve timing gear.

Answer 2

Water getting sucked into the engine. That's the only thing I can think of other than camshaft timing being off, which is far less likely.