How bad would it be to dye my hair before I went to the beach?

So my mom booked a toner appointment for my blonde hair two days before we go to the beach, i'm not sure if the toner will get messed up by the salt water... i also planned on doing some box dye colors either pink or blue underneath. I would like to do it before I go to the beach. Because it would look cool, but I'm not sure if something would happen if it got wet Or was out in the sun for too long?