The RSGB Guidance Resource page provides links to useful amateur radio licence guidance

Foundation licence holders can operate from sailing vessels (with optional /M suffix) but only on Internal Waters such as The Solent or Bristol Channel.

The RSGB provides a link to the UK Territorial Sea map which shows these Internal Waters.

RSGB Guidance Resource - http://rsgb.org/main/operating/licensing-novs-visitors/uk-licensing/guidance-resources/