Like everyone else said, use Modest.
It's the best nature for Kyogre, hands down.
Kyogre is meant to be a special sweeper, since it has godly special attack, and you should always try to nourish and mainly focous on your pokemon's strong points.
Timid can work, but modest is MUCH better. Kyogre doesn't really need to be fast, it already has uber defenses to survive attacks, and it'll hit the foe so hard with special attacks that they'll probably die in a hit or two. (Unless your foe's a Blissey, Regice, ect. then it'll take more)
Still, go for a modest one.
EDIT: Sorry, your post beat mine, I didn't see that you already decided on modest...
I realy like the modest idea. Thanks. What are the chances of a quick claw striking first?
It's only 10%, so it's not a reliable item. Leftovers are usually a much better option for a hold item.