Depends on how far out to sea you’ll end up going. I was surprised how far out to sea my wife and I got before we lost signal.

As Tom mentioned there’s also a likely chance your phone will ‘auto connect’ to a satellite connection, for us it’s Telenor Maritime. And with that can come some hefty bills!

So in addition to setting the bill cap to zero (as per Tom’s post), please also look at these steps:

Via the iD Mobile App → Bills → Mange bill cap:

• set the bill cap to £0 and clicked save.

Also via the iD Mobile App→ Plan → Manage your iD services:

• turned International off

• turned Roaming off

• turned Premium off

• clicked save

The steps above stop the billing system from charging you for roaming and with roaming also turned off it’s an added step in preventing any chance of auto connecting.

Also on our (Samsung) phones we turned roaming off, turned off auto network connection and moved it to manual network connection, both via the phones settings. This caused the phones to ping with an alert to say emergency calls only, whereby we then knew we’d left the iD network and they didn’t auto connect to any other network (but more importantly satellite)

Clearly when we got back we reverted most of the changes back.

HTH

edit. If you do decide to do the manual network selection, then change this just before you’re about to cast off. After it scans for networks pick iD. When you get back with your catch simply set it back to auto and it should also revert on to iD.