The 2 main screw holes on the mounting bracket should be slotted so you can adjust the depth of the transducer, there will be one screw you'll have to remove to let the bracket slide up or down. Fill the 3rd screw hole with a good marine grade silicone caulk or epoxy. The 2 screws in the slots will hold it until you get it tweaked out up or down, then when you're happy with it redrill for the 3rd screw.
Tilting the transducer will give you a false depth reading because it won't be shooting down directly under the boat, it'll be shooting at an angle in front of or behind the boat (out towards Aunt Nancy's house somewhere).