Depth ONLY - round 2" gauge that goes in the dash with a transducer that mounts on the back of the boat at the water line $119 - and less likely to get stolen compared to a GPS/Fish Finding unit - Cut a hole or reuse an existing gauge slot that's not purposed or ditch a silly one you never look at and throw this in there. Ebay: https://www.ebay.com/itm/DIGITAL-DEPTH-FINDER-MARINE-GAUGE-HUMMINGBIRD-HDR-650-WATERPROOF-CUSTOMIZABLE/382917708467

Amazon: https://www.amazon.com/Humminbird-407860-1-Digital-Discontinued-Manufacturer/dp/B00417HG3W/

Alternative - in the same price range would be a dash mount Fish Finder / Lowarance Hook Series of depth + sonar kind of item with a transducer that goes on the rear / bottom of the boat - Ebay: https://www.ebay.com/itm/Freshwater-Fish-Finder-Saltwater-Electric-Boat-Fishing-Depth-Locator-Tracker/123733498253

Amazon: https://www.amazon.com/Lowrance-HOOK2-4X-4-inch-Finder/dp/B077PDL6GR