Most countries currently require you to show proof of a negative Covid test before entering the country. With regards to ferry crossings, this means you need to provide evidence of the negative test results before boarding your ferry, and you might be denied boarding if you can't provide this.
The requirements, such as when you need to take the test and the type of test accepted, are different for each country and can change on short notice, so before you travel, please make sure you review the requirements of any country you are going to travel through.
The links below have information on each countries' Covid travel restrictions and entry requirements.