Q. (See question 1810 above) If one ate a bread sandwich while in the lake and close to the shore. (he washed his hands netilas yodaim, by being tovel them in the water), should he bench while inside the lake since exiting would be considered shinui makom?

A. Horav Shlomo Miller’s Shlit’a opinion is that it is better and proper to recite birchas hamazon while sitting outside the lake and being properly covered. Since he can still see the place where he ate, it is not considered shinui makom or abandoning the original place.
The Rov also pointed out that Shulchan Aruch (O.H. 183: 8) and Mishna Berura (ibid. 30) compare birchas hamazon to the amida, in regards to hefsek and the respectful and proper way it should be recited.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a