Depends on if you like lag or not.
You see fluid updates take processing power. Now with in is like a dozen or so its not so bad. But when you start to pump out large bodies of liquids removing 1 block can create a cascade of updates and this only gets worse the larger your drained are becomes.
On the upshot a ocean does not also do lighting updates like lava does. so it should not be as bad.
Hmm, and without counting the drain itself, would you say I may have better performance with or without the water?
I like the idea of a waterless ocean mountain-y landscape, but if its gonna cause performance issues during and after the drain, I might as well just move somewhere else.