The survey method needs one purpose-built sensor and a handful of things most divers already own. The idea is simple: a castable sonar floats and scans the seabed while a phone topside records the track, and you carry a camera below for the optical half. Nothing here is exotic.
What follows are ideas to explore, not a parts list to copy. Apart from the sonar, treat every drawing as "something like this" — you almost certainly have a workable version already, or can choose one that suits your diving. Verify specifications, ratings, and safety yourself before you build or dive anything.
The rig is three sub-assemblies. The sensor is the castable sonar that floats and scans downward. The topside brain is a phone running the app inside a clear waterproof case — the phone talks to the sonar over the sonar's own Wi-Fi, which travels through air above the surface but not through water, so the phone has to stay at the surface within range. The tether and float keep that phone dry, antenna-up, and tracking alongside the sonar while you dive below. The optical gear — camera and a scale reference — rides with you.
The one component you can't substitute. A castable sonar that floats on the surface, scans the seabed beneath it, and broadcasts to the phone over its own Wi-Fi. The whole method is built around it. See the manufacturer →
Holds the phone and a slim battery topside. A clear lid — such as the Pelican M40 (17.3 × 12.7 cm outside) — lets you confirm the app is running without opening it. A compact case is the point: small enough to secure to the float without overhanging or unbalancing it. The case's own IP67 seal does the waterproofing; check your phone fits the 15.2 cm interior length.
Runs Fish Deeper in Boat mode. A dedicated older phone is worth considering — so a flooded case doesn't cost you your daily driver. It pairs to the sonar's Wi-Fi; nothing leaves the case but the wireless link.
Keeps the phone alive across a survey. Slim is the constraint — the case interior is shallow. Mind the heat caution above: don't seal a battery that's hot or charging hard.
A freediving buoy — such as the Cressi Freediving Buoy (72 cm across, ~2.2 kg, dive flag included) — gives you a stable, highly visible surface platform with handles and D-rings. How you secure the case to it — strapped to the deck, in a pocket, or lashed through the D-rings — depends on the buoy you pick; keep it centred and low, and keep the clear lid viewable. The buoy isn't a dry box, so the case's own seal does the waterproofing.
Gives you a reference for distance and direction back to the float — held in the hand, not clipped to your gear, so it drops free if it snags. A finger spool such as the Apeks Lifeline comes in set lengths (15 / 30 / 45 / 60 m) — choose one comfortably longer than your working depth, since the float drifts and the line is held with slack, not taut. Keep it spooled, with a cutter reachable.
The optical half of the survey — without it you only have the acoustic side. An action camera or housed compact that shoots overlapping stills or video for photogrammetry. You likely already dive with one.
A rigid object of known, measured length, laid in frame so the model can be scaled and checked — the physical object behind the Accuracy tool's scale-reference figure. A composite folding rule such as the Perfect Measuring Tape FR-72 (6.5 ft / 2 m) suits it well: fiberglass-reinforced, stainless springs, engraved markings, and rated for wet use — it folds compact and reads clearly in-frame.
Optional marked line giving a sense of scale and a swim guide on the bottom. Mind entanglement: keep it short and spooled, a cutter within reach, and prefer a rigid rod where you can — no contact with living reef.
The float keeps the phone dry and the antenna above the waterline; the sonar floats alongside, scanning down; the Wi-Fi link only works through air, so the float must stay within range; and the spool is held in the diver's hand as a reference for distance and direction back to the float — never clipped to the diver's harness or BCD, so it can be dropped instantly if it snags. The buoy shown is a high-visibility freediving float such as the Cressi Freediving Buoy. Schematic only — proportions are illustrative.
Happy to talk through what's worked, what hasn't, and what to watch for before you assemble your own.