Signed into iCloud, Sail Tactician backs up sessions to your private CloudKit database — invisible to us — and preferences (boat, units, race views, countdown defaults, wind tracking, water lock) via iCloud key-value storage. Install on a new phone or iPad, sign in, and it all appears. A 30-minute race at 1 Hz is about 180 KB, and the first 1 GB doesn't count against your iCloud quota.
Settings → iCloud says whether races are reaching iCloud, on both the iPhone and the watch; if they aren't, the same message appears as a banner on the Logbook. Nothing is lost either way — races are written to the device first. The two things that stop sync are being signed out (sign in and come back; no restart) and, rarely, the app failing to open its iCloud storage at launch, which needs a restart and says so. Being offline isn't a problem and the indicator won't nag about it; races recorded out of range sync when you're back in signal.