Sweeping Blow refuses to target valid targets
Sometimes, targets begin in "invalid" squares (i.e. squares that you can't move into). Most attack powers allow you to attack them anyway. However, sweeping blow won't let you target these squares. (At least, I'm assuming that's what's going on -- I just tried when I was on the edge of some water and I couldn't target any of the targets that were on the water -- I could only target the trio of land squares.)
AdminDerek (Manager, 10x10 Room) commented
You're right about what's happening: Sweeping Blow and some other abilities that affect an area don't allow you to target inaccessible squares. The intent is to prevent those abilities from working around obstacles that should block them; for example, you probably shouldn't be able to Sweeping Blow through a pillar in the middle of the arc. Here, though, it makes less sense since the squares are inaccessible for another reason. We'll think about a better way to implement these sorts of situations in the future.