Part of the convention Sound and light signals
You may make any light or sound signal that cannot be mistaken for a prescribed signal, or direct a searchlight beam towards the danger in a way that does not embarrass other vessels. A light used to attract attention must not be capable of being mistaken for an aid to navigation. High-intensity intermittent or revolving lights, such as strobes, should be avoided.
At sea it looks like this
A yacht that sees she is not being noticed may light up her own sail with a torch: sensible and allowed. Flashing a powerful strobe at the other bridge is not.
The rule says«If necessary to attract the attention of another vessel any vessel may make light or sound signals that cannot be mistaken for any signal authorized elsewhere in these Rules, or may direct the beam of her searchlight in the direction of the danger, in such a way as not to embarrass any vessel».