You don't need a sonar to fish a drop shot. Study the lake map for the area and look for points where both sides and the front of the point drops off into deeper water. Try to position your boat over these areas. I use a 7' rod. If you hold the rod handle just above the water and point the rod tip to the sky, you can get an idea how high 7' is. Open the bail, point the rod tip to the water and raise the rod tip up approximately 7' to the height you just measured/eye balled. Watch the line as it sinks to make sure that the line does not stop...if it does, a fish hit it on the way down. Close the bail and set the hook. If you do this procedure 4 times (4 x 7' = 28) and you are at 28'. Keep the rod at a 90 degree angle to the water (best angle to feel/detect a bite and watch the rod tip). After a few minutes if you get no bites, open the bail, and let out another 7' (now at 35'). Wait a few minutes. No bites, give the reel 6 full turns. Now you are at 26' (35' - 9' = 26') (if 18" equals one full reel turn). If you get bit or catch a fish, remember the depth and try to triangulate your position or drop a marker bouy. Use that depth and spot in that area and the same depth in other areas. Pretty soon, you will get the hang of it.
Good luck!
A fish doctor is a 4" soft plastic bait made by Zoom. Good for Carolina rigs, shaky heads, split shot rigs, Texas rigs, etc. Green Pumpkin is my color of choice.