Shipwreck...I am not sure why you are so concerned. All the stuff that has been covered is in fact very basic.
Couple of principles: Crawl, walk, run, KISS (Keep it Simple Stupid) and plan the dive and dive the plan
First thing first...hit the internet and attempt to gather as much info about the site as possible. You might even be able to find a depth chart. If your friend knows the lake from a fishing use his experience from a depth and GPS perspective.
Crawl, walk, run and KISS. First dive...do it from shore. I assume you have a compass...if not get one and learn how to use it. You could do a straight line out and back, a square pattern or a triangle. Set max parameters such as depth, time per legs or turn time based on PSI and plan to get out with a reserve that you can then deplete in the very shallow upon your return to your entry point. In addition to compass, lights, cutting devices and SMBs are a most. Unless diving a rocky bottom...use proper finning technique and stay away from the bottom. Expect sediments and obscuring phenomena if stirred. That's it. When diving a lake, expect thermoclines. If diving relatively shallow (less than 60 ft), you could expect one or two. Lakes also tend to get darker as you go deeper. How dark...I have seen night diving dark in the middle of the day starting at 40-50 ft on the way down.
As you gain knowledge about the lake, explore other areas. If you go by boat, it is always preferable to have an extra person tending the boat. Depth finder will provide you with depth and contours.
Here you could do a couple of things...anchor the boat and use the mooring line to come up and down. The problem resides in the fact that unless you are diving a specific feature of the lake, as you get down it will get darker (remember what I said earlier) and you may have problem getting back to the line unless you use ...a reel and/or install a white strobe light on the line at depth. You could use a finger spool of 100 ft and do various out and back or if using a strobe light, take a bearing and swim in a specific direction and before the light disappear, turn back, return to the line then repeat the process using a different heading, etc. That way, you will still explore the area but in a clover leaf fashion.
The second method is to go down, preferably on a line, and start swimming in a certain direction or following a depth contour and towing a dive flag or SMB with the boat following you (however to do that you need somebody who is well versed in operating said boat). When you surface, the boat is nearby and you can either swim to it or it will move closer and then you can get back in it. Safety equipment remains the same shore dive...compass, lights and cutting devices.