Right, okay — so, if I'm getting this correctly, he's basically seeing through the eyes of a third party. I'm going to say, use it on this occasion, but you'll probably have to utilise a mermaid (maybe one of those who went farther ahead), since he won't see an animal/bird to cast his spell, I think?

The mermaids probably wouldn't get close enough to a large vessel to see all the small boats (they'd wait for the prey to come to them, why bother?), but Anduin's spell target could possibly see at least four row vessels. The passengers look both living and human, some six-eight per boat. There are more boats at a larger distance, but the mermaid wouldn't get close enough for Anduin to receive a reliable tally.