hmmmmm
if you had a sand bed i wouldnt worry abotu wacking it alittle underwater, i would worry about knocking of rocks or breaking something else, if you go underwater take it SLOW
maybe if the colony is too big or is on a rock you cant remove, try to start at the edge of the colony and try to pry it off the rock with a small screwdriver.....
but the best would be to take it out, take a hammer, put it between the individual heads if they have em....if they dont (like favia dont) just wackem at the point where you want the break. i would always have at least one "full" head.....i always feel better that way.
use a hammer and a chisel...or better yet use a small screwdriver.
individual polyped coral, like acan, is alot easier.....you can wedge a small screwdriver between them and wack or even pry it off.....
note, when you go to pull em out of the water, use your fingers to "flick" water or wave around the heads so they withdraw their polyps......