Firstly: Be aware you won't have mobile network availability most time on a cruise.
If you pick a Vodafone "Callya" prepaid SIM card in a German harbour you can book attraktive roaming packages e. g. "
Reisepaket 2 Wochen":
You get 700 mins talk time, 700 SMS, 700 Megabyte (valid in EU member countries and Norway)