mmm ive had my rs1800 in water that deep before but only for a few seconds at high revs and that was with a cone filter Water was only at the bonnet level for about 5 secs so depending on airbox placement he MAY have got away with it. As for exhaust issues as long as you have engine revving tits off i dont see the problem?? I used to take my RS1800 through deep puddles and fords occasionally for a laugh and found with water covering the exhaust the revs died unless you floored it and held it floored but it wont die unless you let the revs fall a full bore of water into the airbox would be terminal though for the reasons mentioned by others

Interestingly my RS needed loads of floor welding come MoT time and i founf that out cos water used to come in in deep puddles

Wether it was done properly or rigged/fake on top gear we wont know unless some insiders find out. I can imagine them having spare cars just in case.