Ohhhh... The writers should have appeased the vast population of volcanologist Star Trek fans in the audience with an obscure fact. All 47 of them...
Got it.
It's hardly an obscure fact that when you combine sulfur gases with water, you get acid.
Given that Greg Cox addressed this issue in his Khan novels, I'm assuming
he thinks it's worth mentioning (and I'm quite sure that Mr. Cox has more than 47 fans).
All I could think of when nuKirk and nuMcCoy jumped into the water was that they were really jumping into acid... and nobody mentioned that fact.
Um, had the volcano actually erupted yet? At least enough to create massive lava flows into the ocean that would acidify the nearby water?
Oh, and this from the USGS in response to FAQ about the behavior of volcanoes in movies:
Q: Can lakes near volcanoes become acidic enough to be dangerous to people?
A: Yes. Crater lakes atop volcanoes are typically the most acid, with pH values as low as 0.1 (very strong acid). Normal lake waters, in contrast, have relatively neutral pH values near 7.0. The crater lake at El Chichon volcano in Mexico had a pH of 0.5 in 1983 and Mount Pinatubo's crater lake had a pH of 1.9 in 1992. The acid waters of these lakes are capable of causing burns to human skin but are unlikely to dissolve metal quickly. Gases from magma that dissolve in lake water to form such acidic brews include carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride. However, the movie's rapidly formed acidic lake capable of dissolving an aluminum boat in a matter of minutes is unrealistic.
The movie mentioned in passing in the above quote is
Dante's Peak.
It takes time for the water to acidify. And, we're talking about relatively small crater lakes, not stuff being blown or flowing into an entire ocean (which hadn't happened, anyway).
Full link:
http://volcanoes.usgs.gov/about/faq/faqmovie.php