Einstein's Impossible Point of View Frame of Reference.

Ok . I am standing on the shoreline watching a ship go by and I see a rock falling from a mast to the floor of the ship.  I am pretty much standing still verses the ship being on shore.  I  see the rock falling in what is an arc as the ship moves forward. On board the ship my video cam shows the rock falling straight down. Both points of view are true and real.   OK.   But then there is Einstein's thought experiment with the rocket or the elevator and laser light going from one wall to the other perpendicular to the acceleration of the elevator or the rocket ship.  Where am I able to stand still and observe that?  Ok so skipping ahead I guess I go out and look for black holes?  Even so my stationary point of view is impossible except as that mental experiment. I have undone this problem in the past with the simple observation that space has no built in metrics  . Einstein ultimately makes the speed of light as a constant in a vacuum the metric of space and time.    That is especially an impossible metric point of view certainly because light can only be assumed to be  in existence between two objects where as it actually might not be.  That's the box with the cat that is both dead and living at the same time.  I am not saying the speed of light has not bee measured accurately or observed but I am saying that the observation with light gets tricky where you are at a distance observing a flash of light on one wall of a space ship coming from the other.   We can test the theory by repeating an experiment with light multiple times and we know the probability of sending light where we direct it so long as there is a detector a reflector for our distance observation.