Very good points on dredging by Paul and Emilie and I would add a couple of tips.
 
I always attach the tow rope at about midway on my small boat, it prevents the
stern being pulled down and also in case the dredge gets stuck, this will be
indicated by the boat trying to move in a circle.
 
One other gadget that I have, should the dredge get really stuck, it is a last
resort, but always worked, especially if the dredge gets stuck at a few metres
from the bottom of a deep reef or a large boulder.
I use a messenger. This is made by filling up an old motor scooter tyre with a
cement mixture and is therefore a bit heavy. It is used by inserting the dredge
line through the hole, tying the line to the boat and with the line a little slack
letting it slip through to the bottom, like a heavy ring in a chord. When the
messenger hits the dredge it usually frees it first time and both dredge and
messenger can be retrieved.  When the depth is only at a few metres, 30-50 I
usually tie the messenger with a second chord and let it down slowly through the
dredge line. With this method there is the advantage that if the dredge is not
freed immediatly one can lift the messenger a few metres and bang it down again. I
use this method frequently also to dislodge and retrieve stuck fishing lines or
nets and it does miracles.
Constantine.