Where burial at sea off the coast of England is proposed, an application for a FEPA licence should be made to the Marine Management Organisation (previously the Marine and Fisheries Agency), a subsidiary of DEFRA ( Department for Environment Food and Rural Affairs). The licence normally takes the form of a letter and where granted, will be issued free of charge.
Such licences will include the date and the location at which burial must take place, and other conditions, including specifications of the materials and special design of the coffin be used (to ensure it sinks quickly, stays there, and does not contain any materials that may present a danger to the marine environment). Bodies which have been embalmed will not be allowed burial at sea, for similar environmental reasons, as this preservative could cause water pollution.
Amongst other information, the application will need to include a certificate from the appropriate medical practitioner that the body is clear of fever and infection. The Coroner may also need to be informed of the intention to remove a body out of England as prescribed in the Removal of Bodies Regulations 1954 (as amended).
Lastly, (besides the usual things that need to be organised for a funeral), you need to make sure that the boat you charter to take you out is big enough to carry the coffin and those attending the committal, and has up-to-date positioning technology on board, so that it all happens in the correct location. If a service of some form is to take place, bear in mind that if the waters are a bit choppy, the boat will probably not be the most comfortable place to hold something like that, and so either on shore prior, or after committal, is probably a better bet. Concerning the weather, one last thing to consider, if it's too bad to sail, the whole thing can be called off.