The EU Directorate-General for Taxation and Customs has confirmed that:- 

1. "According to the Community excise legislation and the jurisprudence of the European Court of Justice, in case of fuel transported in the normal fuel storage tank, the excise duty is chargeable in the Member State of acquisition according to the national rules. It falls upon the Commission to enforce the compliance of national legislation with Community law; and
2. People can acquire red diesel in the UK and use it legally in another member state of the European Union, if transported in the normal fuel storage tank."

The EU Directorate-General for Communication has indicated that the onus of proving that full duty has been paid remains strictly with the purchaser.
The RYA recommends that recreational boaters visiting other EU member states:-

1. Keep receipts for diesel purchased in the UK, to prove that it was bought in the UK, and request that your retailer marks them "duty paid". You should also log the date of refuelling and engine hours to reinforce these records;
2. Do not carry red diesel anywhere other than in their craft‟s main fuel storage tanks; and
3. Do not purchase red diesel in a country where the purchasing of rebated fuel by leisure craft is prohibited by law

www.hmrc.gov.uk/briefs/excise-duty/brief4908.htm