What would happen if the pacific and Atlantic oceans would dry up ?

a lot of fish would die...

Well... something really bad would have had to happen for that to be the case. That's A LOT of water. It's not stable for that much water to be in our atmosphere, and there just isn't enough sediment in the world to hold all the sea water in formations.

So, if they were to dry up, it would either mean our atmosphere decreased in pressure and it all evaporated and was lost to space (possibly like mars), or our atmosphere changed to an extent where the water could be stable within it (kind of like venus).

Either way, it's pretty much assured destruction of life as we know it.