As a number of people have pointed out, you have a plethora of options. Consider the advantages and disadvantages of each option though.
A separate depth gauge, properly mounted, will give you an instant, constant, and non confusing picture of what is going on. You will not need to change screens, risk misreading the information on the
screen, etc. I always have a separate depth and temperature gauge and warning light so I don;t lose situational awareness.
The disadvantage of an inexpensive depth gauge is that it only provides you with limited information (some do include
water temperature and alerts as well as keel offsets).
A fish finder/chart plotter will have the advantage of providing additional information that will assist you in different situations. For example, the Garmin 44DV or Strikers can provide you with a
photo like image of the bottom that will greatly assist you when
anchoring (others do the same but call their systems names other than Downview).
A disadvantage of using a fish finder/chart plotter for your primary depth gauge is that you risk losing situational awareness. Pilots and the military call this information overload. Many of the chart plotters these days, and to a lesser extent the fish finders, provide so much information that you can easily lose sight of the depth.
There are plenty of good and inexpensive depth gauges. In particular, I like the Norcross hawkeye D10. They are between $80 and $110 depending upon who is selling and when (I think they are around $140 with H2O temp readings) and are easy to install. The D10 includes a transom mount transducer but it can be mounted in the hull. Be sure the hull is solid and that there is no turbulence where it is mounted.
There are plenty of good and inexpensive chart plotters and fish finders too. Many companies are selling some Garmin 44DVs and 54DVs (and larger units) at clearance prices since there is a newer model coming out. Don't worry about it being outdated. The difference is not in the unit but in the transducer. The
software in the unit can easily be updated to handle the new transducer if you desire but it is not needed.
If
price is your consideration, get the depth gauge. if
price is not an option, get both. Good luck with what ever you decide.