We have a Dept finder on our boat. I highly recommend one. It tells you at all times the dept of where you are at and if you are coming up on something (well the one we have does) They range in price from $200 and up. You can get them online or at a local boating store.
My husband seconds the dept finder, it's the most important thing on the boat, if you plan on keeping it around very long.
As for the Vibration, the prop could be bent. Tell us more about it, is it alloy or Stainless Steel?