On the boat from Siem Reap I had to do all the journey on the roof, under the bright sun. I didn't care taking sun lotion with me, as I am used to the sun and don't really care much about that.
But 5 hours under that sun was too much for me, and my face got seriously burnt (I peeled away next week).
So at least bring some hat or umbrella to protect you...
The monsoon time in Cambodia is April - October. April is also the hottest month while December and January is about the coolest. Always make sure you drink plenty of water in the heat.