Simply having the phone with you won't cost you anything abroad. Taking photos won't cost you anything although you should possibly check that your phone isn't automatically backing them up to your Google account, as that requires an internet connection to transmit the data.
Making calls or using mobile data abroad will be very expensive with Tesco Mobile though. Anything that connects to the internet, including WhatsApp or your email app, will be charged for. You can, of course though, seek out free wi-fi in cafes and bars; you don't have to stick to only using the wi-fi on the ship.
Tesco Mobile is currently one of the worst service providers when travelling within the EU. (Outside of the EU, things can be very expensive with any company). For example, calling from the EU to the UK with Tesco Mobile costs 25p per minute for pay-as-you-go customers or 55p per minute for contract customers. Asda Mobile effectively charges nothing at all for pay-as-you-go customers using 'bundles' (since they can just use their normal UK call allowance which, for most 'bundle' customers, is unlimited anyway) and only 4p per minute for other customers.