Much has already been said, but the system I use on new bodies of water.
1). While at home study old reports on the lake.
2). Look for maps of the lake, there is a lot of stuff on the internet. In Florida the U of Florida has topo maps of all the lakes.
3). Prepare the boat and tackle for what you think would be good.
While at the lake.
4). Ride around slow looking for structure, depth changes, boulders, flats, points, in the area.
5). Look at the bank, you can tell a lot about whats below by looking at what is above. Look for blow downs, steep slopes, cliffs, rock slides.
6). looks for bird activity, happy water (shimmering water on the surface), fish chasing bait.
Now time to fish.
7). Remember you not only have surface area to fish but explore the whole water column all the way to the bottom. I will begin by fishing a bait like a fluke or senko fast at the top of the column, and then slow up and let it slowly fall to different levels and then fish it back. Sometimes they will be mid levels suspended or even at the bottom.
8). Once you find them then go to baits targeting that depth.
Being in Florida deep water for us is usually around 20 feet deep, I would never fish deeper the 30 feet here because of reduced O2 levels from all the decaying vegetation that ends up deep.