file.rename
will rename files, and it can take a vector of both from
and to
names.
So something like:
file.rename(list.files(pattern="water_*.img"), paste0("water_", 1:700))
might work.
If care about the order specifically, you could either sort the list of files that currently exist, or if they follow a particular pattern, just create the vector of filenames directly (although I note that 700 is not a multiple of 30).
I will set aside the question, “why would you want to?” since you seem to be throwing away information in the filename, but presumably that information is contained elsewhere as well.