The Maritime and Coastguard Agency (MCA) has defined the legal responsibilities of small recreational craft here. If boats are to be used commercially, and if they carry less than twleve passengers then they have to comply with Codes of Practice. These Codes effectively exempt small craft fro the provisions of the Safety of Life at Sea Convention (SOLAS) which is applicable by international agreement to all vessels. Note that the MCA has introduced the Harmonised Code, ie one Code that replaces the four codes referenced above. Legally though, the Harmonised Code is not yet in force.
All boats "put onto the market" in the European Economic Area have to comply with the Recreational Craft Directive. Follow these links...
RYA Introductory Guide - read and answer ten unseen questionsExclusions from the RCD
Comprehensive list of standards
Guidance as to the technical documentation that is required from the boatbuilder
Example of Owner's Manual that the boatbuilder must supply
Examples of the Declaration of Conformity that the boatbuilder must supply
What happens if you want to import a boat into the EEA
Link to Seafish standards of construction for fishing boats
Info on calculating the scantling numeral
ISO 12215