Short answer
Reference 210.30.42.20.01.001 is Omega's Seamaster Diver 300M in 42mm with a black dial and steel bracelet. When a seller advertises a "hippocampus" replica of that number, they mean a counterfeit of that exact watch, named for the seahorse stamped on the caseback. What you get depends almost entirely on which factory built it, not on the reference number printed on the listing. Two watches can share the same number and differ by hundreds of dollars in finish.
What the reference number tells you
The digits break down into case size, material, and dial code. On this reference that means a 42mm steel case, 300m rated in the genuine version, a black ceramic dial with laser-cut waves, a black ceramic bezel with a white enamel dive scale, a date at 6 o'clock, and a helium escape valve at 10 o'clock. Lug width is 20mm, and the case sits roughly 13.5 to 14mm thick depending on the version. If a listing claims 42mm but shows a 41mm case or puts the date at 3 o'clock, you are looking at a different watch wearing the wrong label.
Why "hippocampus" is the nickname
Omega has used the seahorse on Seamaster casebacks since 1957. On the modern Diver 300M it is stamped or laser-engraved in relief, and it is one of the first details people photograph when they compare a replica against a real one. The engraving should have depth and clean edges. A shallow sticker-like seahorse is a reliable sign of a low tier build.
What separates the tiers
- Case finishing: good versions alternate brushed and polished surfaces with crisp transitions. Cheap ones blur the line between them.
- Dial: the waves should be cut into the ceramic and catch light at an angle, not printed flat.
- Bezel: the dive scale should be smooth, evenly filled, and rotate with firm clicks and no back play.
- Crystal: sapphire with a proper anti-reflective coating on both faces. Mineral glass shows up as a cloudy edge under light.
- Bracelet: solid end links that sit flush against the case, a clasp that closes with a solid snap, and screws rather than pins on the links.
Movements you will run into
Most replicas of this reference use either a decorated automatic based on a common Asian movement or a clone of the Omega 8800. The clone looks closer through a display caseback and usually keeps better time, but it costs more and is harder to service. Neither one is a Master Chronometer, whatever the rotor says. If the watch matters to you as a daily wearer, ask which movement is inside before you pay, and ask whether a local watchmaker will touch it.
Water resistance, honestly
Treat any replica as splash resistant at best. Gaskets are not tested to 300m, and the helium escape valve is often cosmetic. Showering, swimming, and diving are how these watches die. If you want a watch for the water, buy a real dive watch from a brand with a warranty.
Price bands and red flags
Cheap versions use plated finishes, hollow end links, and printed dials. Mid tier gets you a sapphire crystal and a decent bracelet. The top tier focuses on case geometry and movement decoration and costs several times the entry price. Red flags include stock photos that do not match the described specs, no photos of the caseback, prices far below the going rate for the tier claimed, and sellers who will not show a timestamped photo of the actual piece.
Before you buy
- Confirm the case size, lug width, dial color, and date position match the real reference.
- Ask for photos of the caseback engraving and the clasp.
- Ask which movement is fitted and whether it is serviced or as-delivered.
- Check the seller's return policy and shipping method, and understand that customs can seize counterfeit goods.
Selling and importing counterfeit watches is illegal in the United States and most of Europe. That is worth knowing before you order, not after.