If using a boat on the sea, especially abroad, then you will need the ICC. This is achieved via a RYA practical training course, or by taking a direct assessment. If on the sea then the training is best taken on coastal waters; you aren't going to learn much on a lake or river... The most common route is by attending the RYA Level 2 Powerboat Course with Boatability. 

Regarding Inland Waterways; here in the UK it is not normally a problem and the RYA Level 2 powerboat course will cover you for both Inland and Coastal waters. (Please Note: by attending a course on inland waters some centres may allow you to apply for a Coastal ICC, but does not compare if doing it on the sea!). If you intend using a boat on the inland waterways across mainstream Europe then you will also need the CEVNI (Code Europeen des Voies de Navigation Interieure). This covers the rules of the road and regulations for travelling the interconnected waterways throughout Europe. This can be taken online or you can do it with us here at Boatability, which could be taken at the same time you attend the RYA Level 2 course.