Often times we find vintage watches watches marked water proof, and other marked water resist (or water resistant). some times we even find what looks to be like two counterparts of the same model but with different water resistance branding. Here is the reason why that happens, as it relates to Seikos. The base info can be extrapolated to other brands as well.

In 1968 the US (NOT Japan) changed the laws regarding water resistance markings on watches. Up until that year watches that were sealed against water intrusion where typically marked as Water Proof. As we have all experienced, no watch can be made perfectly sealed. there are tons of circumstances that can cause seals to fail, especially as they age. the law changed in 1968 and it required that starting in 1969 manufacturers begin changing the marks on their watches from water proof to water resist or water resistant. The change was not instantaneous, but rather gradual and with a final implementation deadline. As I understand it, Seiko began making the changes to their models and had changed the markings on most of their new watches by 1970, with the few remaining exceptions finishing in 1971. From that point forward you don't find any seikos marked Water Proof.

Aaron Costello expanded on my thoughts with the following:

Quote Originally Posted by Cobrajet

Gabe is Right. In fact, it seems that Seiko did not even make the change across the board to all of it's models at the same time. Watches destined for the US market (those with case numbers ending in "9") seem to have been changed first...sometime in 1969. But the "Proof" designation continued on for the SAME models in other markets for another year or so. It seems that 1970 is also the year from which we see the most "Proof/Resist" watches as well, so it was not a change that happened overnight.

I have a July, '69 6139-6009 that is marked "Resist" on the dial AND the caseback, but most of these watches destined for other markets are still marked "Proof" through most of 1970. The same seems to hold true for many other models as well, at least according to what I have seen. The "Resist" designation appeared on US-market watches first. I think Seiko simply complied with the US rules for US watches to begin with, then decided a bit later that they should go with "Resist" on all their watches no matter where they were sold. Perhaps this was due to liability concerns about "Proof" marked watches purchased abroad and brought back to the States. --Aaron
I hope that helps shed some light on why some watches are branded as water proof while others as water resist.