Some of the freighters will take the odd passenger, quite legitimately - have a look at which shipping lines serve the Norfolk-Iceland route, as well as perhaps other Eastern Seaboard ports. Eimskip will be one of them - look up their US contact details and ask. However, even this approach will likely be more expensive than flying. If you are a US citizen then you will be able to enter most/all European countries for up to three months without a visa. In some of these you will be able to extend your stay by applying, but all such leaves to enter will strictly exclude permission to seek employment.