The Halacha prohibits placing cold water on a Blech, since that constitutes cooking. The question is whether on may place cold water or raw food next to the fire, in order to warm it up, with intent to remove it before it reaches the critical temperature of Yad Soledet Bo.
The Rambam (Ch. 22) and Rashi (Shabbat 40b) and other Rishonim allow this, even if left close enough to the fire that it could eventually reach Yad Soledet Bo. They are not concerned that one may forget and leave it to reach the cooking point.
However the Rosh (Shabbat 3:10) cites a Yerushalmi that clearly prohibits placing the water in a place where it could eventually reach Yad Soledet Bo. Rather, one may only put the water far enough that it would warm up without the possibility of reaching Yad Soledet Bo. This seems to be the conclusion of the Rosh, as well as the Tosafot and many other Rishonim. Some Rishonim even hold that Rashi retracted his lenient opinion.
The Bet Yosef (318) writes that since the majority of Rishonim and the Yerushalmi are strict, the Halacha is in accordance with them. Therefore, he rules in Shulhan Aruch (318:14) that one may only place water in proximity to the fire where it can never reach Yad Soledet Bo.
SUMMARY
One may heat up water or raw food by placing them in proximity to the Blech at a distance where they can never reach the temperature of Yad Soledet Bo.
