I don't have SOLAS at hand to make reference to but the general rule is to keep it as close as practicable to the waterline. Lifeboats have to kept as close as possible to the accommodation block (the superstructure) and practically all merchant ships have a deck called the boat deck. This can be the one just above the main deck. On large tankers, the lifeboats are on the main deck itself. When on ballast, these decks can attain a considerable height above the water line as the ship is light. If you notice, the modern free-fall lifeboats are usually located on the poop deck at the stern as this is the closest to the water line.
The attachment below is from the 'Code of Federal Regulations' of the U.S.
