Just like a fish finder finds the depth of the water directly beneath it, Im trying to find a sensor that I can purchase for the Arduino that does the same. would like it to check up to 20 ft at least, with high accuracy, +/- 10 or 15 cm. All the threads and info I've been finding are water level sensors not for water depth. So does anyone know of a sensor like this and where I can find one?
The simplest sensor you can build is a weight on a string. Lower it down until the tension of the string is reduced. I assume you want sonar. So just go buy one they are dead easy to interface to.
You can buy these at marine hardware stores. They are not cheap. Look for a depth finder that has "NMEA" output. You may have heard of NMEA as the data format that GPS receivers use to send location. But it is a common format used by wind, depth and speed sensors.
The NMEA format is just 4800 baud serial ASCII data with end of lines and the data is typically sent once per second. You can read the data in a PC if it has a serial port and a terminal program.
The sensors itself looks like a plumbing fitting. You drill a 1 or 2 inch hole in the boat and install the sensor facing down then a wire runs to the main unit. In theory you could create electronics that "ping" the sensor and wait for a return but it's easier to buy a complete unit with NMEA output.
If you try and roll your own you have to deal with thermoclines, soft bottoms, rocks and even fish and do a lot of signal processing to find a reliable depth. This is not like sonar in air where there is a huge difference between a solid wall and air. Soft mud is not so much unlike water. But a NMEA sensor would be very easy to use. You pay the big price for the DSP.
My favorite store is West Marine. Google it and I bet there is one near you if you live near ocean. I recommend them because of their high level of customer support. They will point you at the best product and answer questions like "what's NMEA?"