There is no need for a visa for a Baltic cruise at present unless you wish to disembark in Russia not on a tour (either Disney or privately arranged). Otherwise you're ok without.
In response to the OPs question, the talk of EU visas being needed by US citizens is because of a lack of reciprocity. The US still won't give 5 EU countries visa free travel so the European Parliament in March this year said unless there was equal reciprocal arrangements they would withdraw the right. They argued if a US citizen can enter all of the EU without a visa, all EU citizens should be able to enter the US in the same way. This is of course for a limited period of time (90 days or less in EU). The European Parliament isn't a law making body however, so any change to this rule would be protracted.