Are there limits to waters energy absorbtion capacity
The heat transfer is limited by the flask material heat transfer rate, which is proportional to the thermal gradient within the flask. The external flask temperature will follow.
On the water side, liquid can jump out of flask in form of the thick foam, powered by the vapour. This is the major limiting factor.
On external side, at high enough power, the eventual plastic holders/features may start to melt/decompose. At higher power, the flask may start shining red. Or start to melt if metallic.