Steve,
I'm assuming you are asking about the dive by the Blue Water Bridge. There are a couple of different dives in that area but probably the most common and most manageable (was going to say easy but with the currents that would be misleading) is what is known as the Monarch dive. Actually two wrecks on the Canadian side just South of the Blue Water Bridge and north of the casino. Here is a post I put up about this dive a couple of months ago:
Gear up point: We usually park almost directly under the Blue Water bridge in a parking lot across from the new walkway that they have built along the river front.
Entry Point: Go to the breakwall by the tall monument (stone monument with a bunch of faces cared on the top part) about 14' tall. At the breakwall by the monument you should see a section of the breakwall that is formed into giant steps bordered on either side (North and South) by sections that are more of a slope. My preferred entry point (and gearing up spot) is the northern edge of the boundary of the "step" and ramp sections. At the entry point the northern eddy is relatively strong so make sure you gear up while in the shallow water.
To find the barge you basically head perpendicular to the current. Since the first part of the current is heading North this will pull you slightly to the North West. The first section of your descent will be rocks followed by a sandy section. At the edge of the sand there is gravel/pebble line. This is the point where you will notice the current has done a 180 degree turn and is headed south. If you aren't on the barge already the join the current headed south and in about 15-30 secs you will come across it (it is at the sand/pebble line). Depth is 30-34' at this point
Current is ripping on the barge with the changes that have been made to the river front so make sure you are looking for handholds when you let go of one before you hit the next. There are cargo holds on the barge that you can drop into and the current is not as strong. You hit the barge on the bow first and then work your way towards the stern. We normally meet on the stern section after about 5-8 mins of run time (depending on breathing rate and size of tanks).
Once you are ready to let go of the barge (you can hide from the current behind the stern), you continue to follow the sand/pebble line. The Sarnia club had a line running from the barge to the monarch along this line but as of Friday night (July 14th) the line had broken so we were back to navigating to the next wreck by follow the sand pebble line. The sand pebble line is actually more of a transition in this part of the river from pure sand to pure pebbles and there is about a 10' wide stretch where there is a mix of the two. I have had the best luck by staying between the pure sand and half way across the transition to pebbles.
About 1-2 mins after letting go of the barge you will start to run into the remains of the line that ran from the barge to the monarch (unless it has already been reattached to the barge - or completely removed since the segments were confusing some divers). After another minute or 2 (around 3 mins from leaving the barge) the depth starts to increase to about 45-50'. Between 3 - 4 mins after leaving the barge you will come across the bow of the monarch (unless the current is really moving - fastest I've made the trip is around 2 mins). The wreck is lying on its starboard side (and if you are two far to the west of the sand/pebble line you may end up running face first into the wreck or slipping by on the port side).
Once on the wreck you can usually spend 10-15 mins exploring it or less if you want to drift all thw ay to the CSL docks. Personally I spend as much time as possible on the wreck - when diving an alum 80 I get down to about 1000-1200 psi. When I leave the wreck I head SE (I try heading east but the current ends up pulling me south at the same time).
Once you get in about 18-20 feet of water the current has let up alot - actually by this point the back eddy is starting to setup and you will find you will be heading north again. about 2 mins later (can be longer or shorter, depending on how far you have been pulled south) you will come across the remains of the breakwater "plate" that used to have a cable attached to it to run out to the monarch. This is a large metal structure that usually has a large number of fish hiding under it from the current.
By this point the current is starting to pick up to the point you can easily drift with the current as you follow the rocks that make up the breakwall (heading north) in about 18-20 ' of water still. About 3-5 mins later you will run into the second "plate" which is once again a large metal plate although it is closer to the ground. I believe you will likely see a line (a rope) running out into the river which is actually attached to the barge. You can either ascend here (and come up just south of where you went in - at the southern boundary of the steps/slope of the breakwall) or continue north for another 15-30 secs and find the 3rd "plate", once again a steel sheet left over from the old river front) and then head directly in to the shore and hit the your entry point dead on. Be careful though since the current is moving pretty quick here so maybe the best bet for you first couple times is to come up at the 2nd "plate" that has the line to the barge attached to it and the drift for 30 secs in 5' of water back to your entry point.
Hope this helps - also, it is highly reccommended to use a compass on this dive due to the switching currents. If you want to meet up for this dive for your first time then let me know since I am willing to dive most nights or weekends (and if I'm not I can get you in contact with others that are experienced on this dive).
If you are looking for details about the Port Lambton dive then let me know and I will post the details about that as well.