Soca fo so said:
Maybe it's something to do with the breathing apparatus he used, maybe the oxygen cannisters? But I've no idea how oxygen and acid would react...
There's a good question-- it says "wetsuit", which I interpreted to mean "wetsuit, and wetsuit only", but typically there's a lot of associated diving gear such as a mask, air tanks, weights, fins, snorkel, etc. Plus it's James Bond, who always has a wide range of other assorted gear like camera pens, pistols, arsenic, and so forth.
Also, earlier, the reply was "nothing apart from him and what he had with him went in the acid", which given the phrasing implies that there was something else he took with him into the acid apart from his normal body (no gadgets, etc implanted), and a material that covered perhaps 95% of his skin.
As far as I can tell, his face is directly exposed to the liquid that's in the vat (that's almost explicitly stated). Hence, by the time his face hits the liquid, the liquid can no longer be a strong acid (or else he's harmed, even slightly). So something has happened to the acid to reduce its strength (or turn it into some other liquid) before he's completely submerged.
The only ways to do that would be to add some other agent (ha-ha!) to the liquid, which Bond either has with him or somehow is added otherwise.
DaveE