There are 50 reef balls in the water off West Harlem Piers and they've been there since 2006. On October 26, 2018 I found some tires near there on the rip rap by the Williams / Transco gas facility that were covered with oysters . The presence of the oysters on the tires (and they were young oysters) tells me that there was a lot of spat in the water. This makes me posit that the nearby reef balls are the source of that spat. I would like to photograph them. I know approximately where they are but not exactly. The tops of the reef balls are supposed to have at least 9 feet of water above them. I have a gopro camera. I have underwater lighting. I'd like to get within 2 feet of a reef ball. It would greatly help to know where they are. I feel that a water depth sensor would tell me this.
I have researched this and there are many solutions on the market. The most sure solution is the Garmin Striker 4. It costs about $100 and then you need a method of putting the transducer in the water. Less expensive possible off the shelf solutions involve the Venterior VT-FF001 or Lucky Fish finder for under $40. Its not clear if these devices will provide depth. All these devices are designed as fish finders. Providing bottom information is secondary to them. The Garmin is nice because it also has a GPS attached which will allow you to note a location. I've heard of some castable depth/fish finders like iBobber, Deeper, Fish Hunter Pro that use the phone screen. They all seem to have issues and are all wireless. The first two use bluetooth and the last uses WiFi. They all disconnect but only the first has the easiest recovery.
Making one from parts is quite complicated due to the fact that mud and water have similar densities. However, the reef balls should really stand out as they are made of concrete.