Hi - I just purchased DivelogDT for my Mac and Divelog for IOS. Both seem to say that they can sync to iCloud. My iPhone and iPad seem to sync together nicely, however, no matter what I do, my Mac will not sync over iCloud.
The macOS syncing is a general "cloud" based sync of which iCloud is just one of the technologies, the others being Dropbox or Google Drive. I'm guessing you already know this, but to sync with iCloud, you just select the "Cloud" panel and select iCloud from the list. If this is the first time putting your logbook in the Cloud then click "Add/Replace Logbook TO Cloud". If you've already put it there and just want to sync, then just click "Two-Way Logbook Sync"
Dive Log for iOS added iCloud 1.0 syncing when it was first available from Apple years ago. DiveLogDT uses the next generation iCloud Drive (which I call iCloud 2.0). The original iCloud does not allow any other external applications to access it's data in the cloud - this is an Apple security feature. So DiveLogDT can not access the iCloud 1.0 files of Dive Log for iOS. We are upgrading Dive Log for iOS to use iCloud Drive, but it is not available yet. You can be sure it is a high priority though.
Also, is there a way to get the sync to iCloud to happen automatically? Seems like any changes that I do on my iPhone or iPad, I have to go through to the sync screen and sync up, then go onto the other one and sync down (then go to my Mac and sync over Wifi).
There is no way to have cloud syncing happen automatically for either DiveLogDT or Dive Log for iOS. This is by design. Divers who dive locally don't really have network issues, but divers who dive remotely do. We want to make sure that syncing happens only when a user initiates it so that they do not use limited satellite bandwidth or expensive internet connections accidentally. Also, network connections in remote dive destinations are notoriously bad quality. And bad quality internet connections invites data corruption. Passing around large logbook files in this environment seems like something a person should decide on, not an application. We may in the future add an option to do automatic syncing under certain network circumstances. But remember, if two devices decide to sync at the same time, then that creates conflicts which requires syncing again, which creates conflicts which etc etc. The good thing about having user initiated syncing is that people are less good at doing two things at once
So yes, for now, you need to go to each of your iOS devices to sync to the cloud and also sync to the Desktop via WiFi.
Sorry we don't have better answers for you,