that looks to be pretty far back for a daggerboard trunk...but I don't see the daggerboard trunk. I believe the teams are allowed to develop their own foils for the 45s...perhaps these guys have some sort of gasket with a drain in the daggerboard case?

I hadn't heard anything about water ballast on these boats.

edit...it's likely that this hole is in the stern scoop just after the joint where the hull's stern meets the main hull. The last 5 feet of the hulls are bolted in-place and removed so they fit in a standard 40 foot shipping container. The stern is open so maybe there is some sort of drain there in front of the rudder because perhaps the hull bottom slopes forward there? I do see a seam running up the side of the hull from that drain hole...so, yeah, this hole is definitely in the bolt-on stern scoop.