These are the best Canon lenses for underwater photography, chosen for their sharp optics, close-focus capability, predictable autofocus, and compatibility with common housings, domes, and flat ports so you can cover reef scenics, big animals, and macro critters with clean corners and minimal backscatter—whether you’re on RF mirrorless or adapting EF glass. For wide reefscapes and large subjects, fisheye is king: the EF 8–15mm f/4L Fisheye USM (in a proper dome) delivers corner-to-corner sharpness, dramatic FOV, and the ability to get close—crucial for clear water, poppy color, and reduced particulate; it adapts perfectly to RF bodies and is supported by a vast ecosystem of domes and extension rings, making it the most reliable big-animal and reef-scenic lens in Canon’s lineup. When you want rectilinear wide without fisheye curvature, the EF 16–35mm f/4L IS USM remains the underwater workhorse for wrecks, architecture, and divers-as-scale frames—excellent edge control in a 180–230 mm dome, filterable topside, and easy to drive with AF; the EF 16–35mm f/2.8L III USM adds speed for ambient-light video and dusky dives, and on native RF, both the RF 14–35mm f/4L IS USM and RF 15–35mm f/2.8L IS USM have growing housing/port support with strong corners when correctly paired with large domes and the right extension. Macro is where Canon shines below the surface: the RF 100mm f/2.8L Macro IS USM is the hero on RF housings—true 1:1, crisp micro-contrast, and reliable AF for nudibranchs, shrimp, and pygmy seahorses—while the adapted EF 100mm f/2.8L Macro IS USM remains a staple with widespread flat-port support and easy wet-diopter stacking for super-macro; on APS-C, the EF-S 60mm f/2.8 Macro USM is superb for skittish subjects at closer working distances or when water clarity demands a shorter lens. For versatile travel kits where you’ll split time between topside and underwater, the RF 24–70mm f/2.8L IS USM can work behind a large dome for big animals and diver portraits when you can’t commit to fisheye, but true macro and true ultra-wide still outperform it beneath the surface; for compact rigs and casual snorkeling on RF, the RF 16mm f/2.8 STM is a tiny topside ultra-wide that some housings support, though dedicated fisheye/rectilinear wides remain the better underwater choice. Practical port pairing is half the image: use a dome port for fisheye/rectilinear wide, match extension rings precisely to lens + adapter length to place the entrance pupil near the dome center, and use a flat port for macro to magnify ~1.33× and preserve corner sharpness; add a flip adapter for +5 to +15 wet diopters on 100 mm macro when you want super-macro, and keep a focus gear/zoom gear fitted so you can work at exact distances without hunting. Technique that delivers: get close (then closer) to reduce backscatter and boost color, place strobes wide and slightly behind the dome to feather light across the subject, and aim for manual exposure around f/8–f/13 for wide (to discipline corners) with 1/125–1/250 s and ISO 100–400, letting strobes set exposure; for macro, start around f/11–f/16 with 1/160–1/200 s and low ISO, favor single-point AF or manual with focus peaking, and rock gently through the plane; for ambient-light video, hold a 180° shutter (e.g., 1/50 at 25p), use the f/2.8–f/5.6 range on wides, and supplement with gentle continuous lights when color drops. Stabilization is helpful for ambient-light video but less critical for strobe-lit stills; disable IBIS/IS if you see viewfinder drift in macro on a tripod base. A simple, high-impact Canon underwater kit looks like EF 8–15mm fisheye in a big dome for reefs and big animals, EF 16–35mm f/4L (or RF 14–35/4L) in a dome for wrecks and rectilinear scenes, and RF or EF 100mm Macro in a flat port with a flip diopter for critters; APS-C shooters can run EF-S 60mm Macro plus EF 8–15 fisheye (adapted) for a light, capable two-lens travel set. Whether you’re nose-to-dome with a turtle, threading through a steel wreck, or stacking diopters on a 1-cm nudibranch, Canon’s fisheye, rectilinear wide zooms, and true macro lenses—paired with the right ports—deliver the sharpness, working distances, and color you need to make underwater images that pop.