Pond building will help burn calories off, EVEN if you have machinery help. LOL Only so much a machine can do. I cut out soft drinks and started drinking zero calorie carbonated flavored water from walmart. Lost 80# last summer! Did very little change to my diet except got away from fried food and started grilling or baking the same foods instead of frying. My heart DR. took me off my blood pressure meds and my Lipids meds this last fall. 80% of my exercise was hauling rock and digging by hand shovels. I must say I feel so much better and I am more active this winter than I have been in years. Now for the original question, If you can go deeper do it, you won't regret it later on. Two factors need to be considered about depth, How cold the winters are and how hot the summer water will get is one factor, Predators are the second. Any pond that is 30" or less and are where there is even a possible chance of a Great blue heron visiting is a invite for a clean out. An adult heron can stand in 30" of water and fish, any thing shallower than that and they can move around to better position themselves to catch fish. 36" deep and their whole body is in the water, They don't like their body in the water and will move on to better hunting grounds if they can't catch fish easy. For temps now would be a good time to check Ice thickness in your area, If the Ice is say 6" thick than 2' of water would be enough void for the fish to survive. If the ice in your area gets 12" thick then you need 4' of depth to accomplish the same goal. A rule of thumb I read somewhere years ago is go 4 times deeper than the thickest ice that will form. IMO, a good rule of thumb to follow. Heat is quite different, Heat warms up faster in small shallow ponds, The faster it rises and falls the more dangerous for the fish, so again, the deeper the better.