Now that we know you're talking about a Socorro trip, I'd say the wider the better. You may think a mid-range lens will enable you to get more shots, and you may indeed get MORE because you can shoot from farther away, but you give up two things (that are FAR more important IMHO):
1. Being able to fit an entire Manta (or two) in the frame when they're right on top of you.
2. On a Socorro trip, being able to fill the frame with mantas or sharks from farther away will NOT (in general) result in photos as good as if you fill the frame with them at half that distance. The reason is that the water is not crystal clear. This is definitely a case where CLOSER IS BETTER, and in that context, WIDER is better...
IOW, I'd stick with the 10.5 or 10-17 for Socorro.