Short answer
Two numbers show up in "fake Seamaster 30 30m" listings, and they do not mean the same thing. A genuine Omega Seamaster Diver 300M is rated to 300 meters, which Omega prints on the caseback as 30 bar. A watch marked "30m" is only rated for splashes, about 3 bar. When a seller writes both 30 and 30m in the same listing, they are copying a bar figure from the real watch and a meter figure from a splash rating. That mix-up is one of the fastest ways to spot a low-effort replica listing.
What 30 and 30m actually mean
Water resistance numbers describe static pressure, not a depth you can swim to. 30 m, or 3 ATM, handles rain, hand washing and a splash at the sink. It is not a swimming rating. 300 m, or 30 bar, is a proper dive rating with a screw-down crown, gaskets built for it, and a case tested to that pressure. Omega rates the Seamaster Diver 300M at 300 m / 30 bar, and that is where the "30" in casual listings comes from.
The Seamaster 300 (the heritage model with the vintage dial) is a different watch from the Diver 300M. Sellers who blur the two often end up with a spec sheet that reads 30 and 30m at once.
Why replicas get the marking wrong
Most replica factories copy the dial and the case shape, then stop. The caseback engraving is where things fall apart. A real 300M caseback carries the hippocampus medallion, the depth rating, and a reference number with clean, deep stamping. Reps often show shallow laser etching, a soft seahorse, or a rating that contradicts the front of the watch. If the caseback says 30 bar and the listing says 30m, treat the whole spec sheet as marketing.
Checks I run on a replica Seamaster 300M
- Caseback: hippocampus medallion, crisp depth text, correct reference, no mismatched numbers.
- Helium escape valve at 10 o'clock: should sit flush, screw down, and line up with the case profile.
- Dial: ceramic wave pattern with laser-cut edges, applied markers, lume that glows evenly.
- Bezel: ceramic insert, tight clicks, lume pip that lines up at 12.
- Date at 6 o'clock: centered in the window, correct font, no magnifier on the modern 300M.
- Movement: genuine runs the Co-Axial Master Chronometer 8800 with a 55-hour reserve. Replicas run a decorated clone, usually with a 38 to 40-hour reserve and a rotor engraving that looks stamped rather than cut.
- Bracelet: solid end links, no rattle, screws rather than push pins on the better builds.
Water resistance on a replica in practice
No replica leaves the factory pressure tested unless the seller says so and can show you the result. Even a well-built rep with fresh gaskets should be treated as splash resistant until proven otherwise. If you want to swim with it, take it to a watchmaker for a gasket check and a dry pressure test. Many will test to 3 or 5 bar without opening the case. Some will not touch a replica at all, so call first.
When a listing claims 300 m or 30 bar on a replica, ask for the test printout. A seller who cannot produce one is quoting the genuine watch, not the one that will ship to you.
Bottom line
The "30 30m" wording is a spec error, not a feature. Judge the watch on the caseback engraving, the dial detail, the bezel action and the movement finish, then decide whether the water resistance claim is backed by anything. Buy the build quality you can verify, and assume splash resistance until a watchmaker tells you otherwise.