I initially put this in here: Ask the guy who is pretty good at SF Story Identification but without success.
Ok so here goes. It’s a novel from the early 2000s and everybody on Earth somehow gets a message basically saying that if you want to go to space, or board the starship or whatever, be within so many yards of a body of water at such and such date and time.
And those who follow the instructions end up onboard an uninhabited alien ship with a very sophisticated AI and a responsive, malleable interior.
On one hand the ship becomes populated with well motivated and well meaning people, while at the same time a number of local levels of government have taken advantage of the situation to get rid of their population’s criminals.