i am a scientist, a orthodox scientist in fact. i can strongly say, negative result is as good as a positive result and in no way it is inferior.
yes, i do agree that if we know that the result is going to be negative then, avoiding such attempt would be wise, but for someother reasons, those attempts are very much required, for example to prove that it is indeed a negative result.
but in this case, discovery of submerged mamllapuram may not be a positive result for "the flooded kingdoms of the ice-age" hypothesis, but this is a positive result for some other theories as they did discover a submerged temple. if there dive was entirely without any discovery, then probably one can say that it was not very usefull, but still, i would say without that you cannot completely rule out the story of sumberged mamallapuram!!
in my openion it was not at all a negative result, in the first place. secondly, it might not help to prove the ice-age civilisation, but......
i think i have said enough. repeatation is also not a good practice for a scientist unless it is critically required!!