We just got back from there in Nov and had no problems getting very close to the mantas and whale sharks and dolphins with our stock 10-17 on the 40D and my 8-15 rolled to 15 on my 7D. I don't think you'll need the 1.4X. These first two are Jeff's images made with the 10-17 rolled to 10mm. I think you can expect the water to be the typical eastern pacific blue with good viz but not fantastic. A lot of the images are also at depth, especially if you get out to Roca Partida. It pays to be close.
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I shot the 8-15 mm about half the time at 9mm
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and the rest at 15mm
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You are going to have a blast!