Diving Log unfortunately cannot handle sub-zero temperature values in the profile data. A profile with just zero values is indeed interpreted as "no profile", because that's how it looks when there is no temperature profile. Here you can see how Diving Log stores the temperature data and why it's not possible to include negative values. That's unfortunately a limitation that cannot be easily removed, sorry.
You can store sub-zero values in the water- and air temperature fields in the logbook. If the temperature profile is almost a flat line, a single value contains at least the same information as a flat line.
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