Dear Rabbi Kornfeld,
How is it possible that one is not chayav for taking water from a boat by setting up walls, even if they are 10 tefachim high and 4 tefachim wide? There is a chance that when you will draw water, the bucket may drift outside of the "Rishus Haychid" due to heavy current, in which case he would really be taking from the Karmelis which should make him chayav?
Thank you for all the Torah and zchus you bring for all of Klal Yisrael.
There are three reasons as to why your problem is not crucial.
1. Seeing as he neither intends (nor even wants) the bucket to drift away, it is a Davar she'Eino Miskavein, which is permitted Lechatchilah on Shabbos.
2. Even if he was Mechaven, he would not be Chayav, since he will be making neither an Akirah nor a Hanachah.
3. A Karmelis is only a Reshus de'Rabbanan, so to forbid drawing water because the bucket may drift away would be a Gezeirah l'Gezeirah.
be'Virchas Kol Tuv.