How much water? Half full? or just a few drop inthe bottom?
It could be that you have a broken seal around the window, and if you can find out the trade name of the window - this should reveal the plastic used - and finally a suitable glue or solvent to seal the edge.
It could be that one or more of the black plastic ventillation plugs has fallen out from the inside of the window - if you remove the plugs and leave that winow in full sun - it should evaporte all the water in a few days. Then replace the plugs.
RECOMMEND HT OU