So you're assuming now that NV ocean is deeper than our world? It could also be shallow for all we know.
What I love about these sort of vs topics is that you create a situation specifically catered for Tobirama to win the fight.
It was already shown in the manga he lost to Madara and FTG was countered easily; and no Tobirama did not place a seal in the middle of the ocean.
If you say it was as easy as you claim, then he would have, but he didn't.
Madara didn't even use a Susanoo in that fight since Tobirama couldn't touch him as it were.
You don't have to defend your fav so fervently
It's a simple matter of being able to use manga phax in correct context.
Technically, EDO Minato & EDO Tobirama are solo gods, because they can throw their kunai upwards continuously. Eventually they'll end up in space. Then they'll just use FTG formula. So the mofos will be stuck in space, and they'll just teleport back to earth.,
So it's just a simple matter of kishimoto being incapable of utilising their jutsu correctly