I'm not sure there will be many boats, GRP or otherwise that will be able to meet all of the OPs intended usage
".............but we think children and adults with challenges would love this safe unique situation to hide from the world for a while and watch and learn about otters, or sit and do simple weaving, painting, sing, cook, .............."
I'd suggest that a portacabin / site office raised off the ground on legs or blocks would be more suitable
I would think that there will be safety certificate requirements to meet/obtain if there is gas or electricity 'piped in' and it is being used by "3rd parties" - particularly those with 'challenges'.