I use a small hole saw bit, and put a hole in the trunk to tie rope to and then attach cinder blocks. Fresh trees sunk slower, and required more weight to get them down....
Lol ya my first expieriance the other day I only tied a half a cinder block to them, chunked them over board, rigger the next one only to turn around and see the first one "bobbing" lol! It takes a whole cinder block to sink one!