[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... ]

That's where it gets hard to back up a belief, because it is based on the scant or even faint evidence that we see today. As faint as it is today and so badly eroded, it's still there.