On a FF camera, I would say the 15mm SIgma would be pretty good.
On the other I also tried a 60mm into a dome that helps you keep a wider angle
I have shot rhinos with many lenses but only DX format see how they go :
With a 100 mm, this is the most useless lens I hate to see a rhinopias when I have my 100mm on. I had three in different ones in Ambon while I was stuck with a 100mm.

Now this is shot with a 60mm and flatport, Better for geting close in Alor
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This one with a60mm flatport was more comfortable to shoot but the juvenile rhino from Padang Bai was much smaller, thus the lens totally appropriate
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This one with a 60mm and Zen minidome

For the wider angles, this is a classic combo with a Tok 10-17 behind a Zen Minidome. I was lucky with 4 rhinos on this dive in Padang Bai.

Same dive using the Tok 10-17:

I sometimes used a Tok 10-17 with a Kenko 1,4 CFWA adaptor behind a Zen minidome, it does a great job for this size of fish, but the AF can be painful. .Here in Ambon.

And a very special combo using the Inon "insect eye" lens, In Tulamben Bali.
