Thanks, all.
Getting close to bergs is a very bad idea, as the drivers of the Zodiacs were at pains to point out whenever we suggested anything adventurous: although it's infrequent, the things can overturn without warning and many of them are jolly big. That said, although I used a 100-400 for perhaps 90% of my photos on the trip, this one was taken at 35mm.
Jeremy