It depends
both are similar. You can ride the ocean though, swim in it even... even without a suit.
Cant do that in space. I would say equally terrifying but there are variables. Space has giants rocks, probably hostile planets, solar flares, black holes, Suns, etc. difference is that it would probably take years just to get close to one.
The ocean is right there... not as deep as space but the largest mass on our planet... half of it is frozen too. On top of that, we really do know that there are creatures living there that could fukk us up in their own element even if we don’t know all of them. And we still don’t know how deep it goes or explored all of it.
So i dont know... both equally scary i guess