Many ferry operators provide satellite-based mobile phone networks on board their ships. If you have data roaming enabled on your phone, it'll connect to these automatically, and you will then get charged at very high rates for any data your phone uses, including any background data for things like Google Maps.
Three's website (this and linked pages) doesn't warn you about this. You're led to think that, for example, if you are travelling to a Go Roam country, you'll simply get charged the £2 per day as long as you stay within a certain limit.
In fact, if you are travelling on a ferry between the UK and a Go Roam destination, or between two Go Roam destinations, or even if you are on a ferry entirely within the UK, if you have data roaming turned on you are liable to connect to and get charged for one of these "maritime" networks.
It seems the ferry companies are allowed to turn on these networks as soon as the ship is a certain distance from the coast.
Your only defence is to have your phone on airplane mode as soon as you set foot on the ship, or to set a zero spend limit. Don't risk turning mobile connectivity back on when you see the coastline of your destination - your phone can still connect to the ship's network.
