Live tracking issue with iPhone?

First of all - all best with your perf, good luck, and good weather!

You did not tell what your Garmin watch is, so I cannot check whether it supports Live Track, but if it does, then the settings for the feature are in Garmin Connect Mobile menu » Safety & Tracking » LiveTrack

You best test it beforehand on a shorter swim, because it is uncertain how well the connection to the phone will work, when the watch is ½ of the time under water. Will you keep the phone on a float, or in the support boat? In any case it will be quite challenging to get a good connection both between the watch and the phone, as well as a solid data link to the Internet on the phone. The boat would have to stay within a very close distance of just a few meters all the time, and even then it is uncertain whether the BT connection can be maintained while you swim. Are you sure there is a good GSM signal and good data connection all along your route? Water is not a good friend with the electromagnetic signal. It may be simpler to track the support boat than the swimmer, so consider it as an option. And test it well before you actually do the swim.

Also, are you sure your watch can keep working all the time of your swim? I do not know how fast swimmer you are, and what the conditions are (crossing the Channel?), but to me it looks like a very long swim of minimally some ~8 hours for elite long-distance swimmers, up to a full day or even more, depending on the currents, weather, and other conditions. I fear that such long swims may be impossible to keep up, for current watches - the power consumption during the swim will be certainly higher than during a run. Again some testing of the battery stamina under such conditions is highly recommended, before doing the real swim.