According to Official Guinness Records,
The fastest marathon dressed as a swimmer is 2 hrs 42 min 24 secs, achieved by Joe Spraggins (UK) at the 2017 Virgin Money London Marathon in London, UK, on 23 April 2017. Joe was the first GWR runner to complete the marathon from over 40 different record attempts.
For a complete list of 2017 records, please visit 2017 Guinness Records in United Kingdom.