Thanks to everyone for the comments.
Just to clarify…
I'm going to be using one of those stand alone hub thingies so that more than one person can access the internet and I don't want to be swapping sims in my phone. The cost is minimal for the device.
I use around 400megs of data a month and as I only spend around 8 weeks on the boat a monthly contract
is a bit of overkill so 'pay as you go' works for me at €15 for 2Gb.
I asked the question because I was told that data only sims are not really that popular in some countries and not easy to come by. I don't want a standard sim that gives 100 mins of talk time, 500 SMS messages and just 250 megs of data
because i would be paying for elemens that would never get used.