@Tim07NOV18 The phone should be backfilled by the watch when they come in proximity. I do that all the time, and it’s pretty cool to watch the backfill process while it’s happening.
As far as the connectivity problem under water, I have no idea. But you won’t get backfill from the transmitter like with Dexcom, it comes from the watch. So if the watch gets no reading it has no data to transfer to the phone.