Best to bring a waterproof marine radio in a dive box on ANY drift dive (less than £100), otherwise an old mobile 'phone wrapped in a plastic bag in a dive box, assuming 'phone coverage. Make sure the boat has it's radio on first (imo solas regs require it to be on at all times, but been on several boats where they hav'ent bothered to turn it on)!
If you are really scared of the dive boat crew's competence you can allways buy an epirb, but they are well over twice this price.
Remember to bring a whistle, smb and when all else fails wave your fins above your head!!
