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@ I ain't Spartacus

Very many thanks for fully and properly answering my innocent question.

No problem. I forgot to mention a couple of other things. One's been covered by someone else - if the submarine isn't near the surface it can't use GPS. If it is, then it has no advantage over a surface ship.

The other is that the ship can regularly drop probes to measure water conditions at various levels. As well as having that device to do it that can't be deployed in rough weather. And that info on temperature, salinity etc is vital to an accurate survey. Whereas a sub is only at one depth at any one time. Although obviously it can change that, but that would affect the survey data.

Finally there's more room on a survey ship. For general gear and comfort of the crew.

Having said that, as I understand it, at least one of our subs did participate in the search for MH370. But I don't know if that was helping with the survey work, or searching for the sonar pinging of the black boxes in the first few days. I suspect the latter, because it was in the early days. We also sent one of the survey ships for some of the search operation.