I have full teak decks. I use SEMCO with outstanding results.
Letting decks go 'grey' constitutes an abandonment to Ultraviolet destruction of the surface wood cells which then easily erode. With the price of teak at now $50 per board ft., without 'sealing, etc.' you will be replacing the deck in approx. 15-20 years, or sooner.
There is absolutely NO reason to keep a laminated teak deck 'wet' with seawater, and the chemical composition of seawater accelerates the erosion of the teak surface.
SEMCO is great but is slowly lifted by 'green water' and therefore needs continuous coatings at 4-6+ month intervals. SEMCO allows the teak to become wetted and thus its usage keeps the 'footing when wet' just a bit under the characteristic unsurpassed 'non-slip when wet' characteristics of a bare deck.
Only real downside is that SEMCO eventually 'transfers' to boat shoe soles and such shoe soles will need to occasionally 'sanded' to regain their 'wet traction'.
Other 'mixtures' Ive used, and with slightly longer 'service time': 1/3 SEMCO : 1/3 Teak Wonder : 1/3 Olympic Deck stain (w/50% carmel tint) ... but 'recoating' may involve 'splotchiness' if worn away by constant 'green water'. The eventual 'transfer' of the mix to boat shoe soles is a bit faster than with SEMCO alone, so you just need to 'sand the soles' a bit more often. Such 1/3:1/3:1/3 mixtures will typically last 2+ years with 'maintenance coats' in the 'worn-off areas'.
Info on the 'mixes': TOG Photos, Projects & Publications
---> FTP download ---> projects ---> decks ---> Teak oil Test/
The best characteristic of both SEMCO and SEMCO 'mixes' is that the teak still 'wets out' when covered with water, not as non-slip as a bare deck but 'close' to the non-slip when wet as bare teak.
All these coatings are easily stripped off with TriSodiumPhosphate (TSP) ... just dilute the TSP/water mixture so that the 'stripping' doesnt extract 'too much' of the tannins/oils of the teak, then bleach the deck with oxalic acid to restore 'color' (I like bleached teak as it keeps the deck 'cooler' on the feet, etc.).
The absolute WORST teak deck coating has to be Cetol or Teak Guard ... great for outstanding slippery-ness especially when wet AND has to be sanded deeply to repair. Cetol also cracks/crazes with age.