I don't know what the water clarity is where you are operating your boat, but in the Great Lakes, where I am boating, you do not need a depth sounder to let you know you are in water that is less than 20-feet deep. You just look over the side and see the bottom.
I prefer a SONAR with a display showing the depth history in preference to just a digital reading of depth. You can usually see a trend in the depth. If I see the bottom is coming up rapidly on my SONAR display, it gives some warning time to the helmsman.
Since you have no electronics at all on the boat now, I recommend getting a combination device that will have
--a GNSS receiver
--a SONAR
--a multi-function display
--embedded digital chart cartography with good detail for your boating area
--a chart plotter
--a navigation computer
--a fuel manager
--a trip log
--and divers other systems
instead of just getting a depth finder.
A multi-function display can be configured to show multiple panels or windows simultaneously, or to show many full-page windows of different information, one at a time, or to show overlays of important information over many of those windows. In general, these modern small boat electronic devices are amazing and are excellent values. Their cost is modest and they deliver an amazing amount of information.
Choosing what device to buy is mostly a matter of your budget. As depth finders, they all work about equally well. If your budget is limited, you can find an obsolete product being promoted at close-out prices that will be a good buy. If your budget is unlimited, then your choices are similarly unlimited. Without some notion of budget, it is difficult to know what to recommend to solve the problem.
The time between new product introductions in the marine electronics market of today is about 90-days. Many new products are introduced or become available around February each year, as this corresponds with major boat shows and fitting out for spring. At the moment, if I were going to buy a new multi-function device for a small boat, I would give first consideration to the SIMRAD GO7 XSE. It has not yet hit the market, but there is some
preliminary information available.
As for a transducer, you will most likely use a transom mounted transducer. A Boston Whaler boat like a 170 MONTAUK is extremely unsuited for a through-hull transducer because of the double bottom hull construction. Generally through-hull transducers are only used on larger Boston Whaler boats in which the hull has been designed and fabricated with a special area, called a putty box, through which one can drill a large diameter hole and install a through-hull transducer. For a 170 MONTAUK you will be much better served with a transom mounted transducer. It will be far less expensive, require far less modification of your hull (which might void your hull warranty), and will work just fine.