You'd want to have evidence, rather than just Faith...
It depends WHY you believe this. There is certainly evidence of localised deltas in some craters, and hence probable crater lakes. The evidence for an ocean is very weak, though.
I suspect there might have been ocean while the planet was still accreting but that as the growth rate (and hence energy input) dropped-off the ocean froze up, essentially permanently.
This is perhaps WAY back before we even begin counting time on Mars - before any cratered surface we can observe, except perhaps the giant impact basins...