Back to Internet in the BVI, this is what I emailed Digi:

""We will be chartering a boat out of Road Town in April and I wonder if you could tell me if our Internet plan will work.

We have an unlocked iPhone 5 that was used on the AT&T network. Our plan is to buy a SIM card from you together with a 3 GB 30 day data plan for $55.

Here is the question. With a Digicel SIM card in my unlocked AT&T iPhone 5 will I be able to configure it as a personal hot spot and allow other phones & pads to access the internet through it? Do you allow it and do you expect the phone to allow it?""

and they replied:

""Yes the plan will work and you will be able to tether. The cost of the 3GB plan for mobiles is $50. The plan that you are referencing is for our mifis/routers and dongles.""

So with sail2wind's comment & Digi's response I have a high degree of confidence it will all work out, just have to wait and see how it plays out in practice for us.