Let me play the devil's advocate: What is the intended use of that signature? Is it more than an additional picture associated to a dive? In particular, if it is meant to certify something, would you intend to make the presence of the signature read only (or the signature disappear once you change the log entry for that dive)? Maybe a signature should at least be for some cryptographic hash value of the dive data.
In the old days of paper logs, many divers had individualized stamps that they use to sign your log. Would you intend to scan those as well?
Here is what I do: In addition of having my dives on the computer, I use the TeX export to print pages for my paper logbook. The template contains a field for the signature of my buddy. That has at least the power of being less editable once printed and signed. But yes, it has the disadvantage that it cannot be done immediately at the dive site, only when I meet the buddy for the next dive.