Depth under the keel for us. Had a problem last year though when we accidently changed the units from metres to feet. Alarm set for 1m but gets annoying going into Bradwell any where near LW.
So no overwhelming majority in favour of either total depth or depth under keel. At least we're aware theres more than one way!
I would suggest labelling the sounder display to make it clear which its set to, but that doesn't work so well with multi function displays, like the B&G I had.
Using alarms doesn't help much imho, as the 'panic' level varies so much. Going into Lymington near LW, half a metre under the keel is good, other places 2m means your not in the channel you should be!
When you get on a different boat, there are so many things you take for granted that can be subtly but significantly different, I need a checklist of 'daft' questions I think!