A simple and effective piece of safety equipment...
I always take a whistle (or two) when I freedive; one attached to me and another to the buoy (tied to the zip puller). I recommend these Slimline Tornado whistles from Acme, in the UK, because they are small and unobtrusive (5cm long x 1 cm thick) and yet also extremely loud. I have stopped a motor boat with a single blast at a range of about 200 metres. I wear mine around my neck on a thin elastic cord (which will break if tugged, so as not to risk strangulation) or around my wrist. Either way, it tucks under my wetsuit or rash vest and causes no discomfort but remains easily accessible.
For the cost of a couple of coffees, a whistle might save your life: Use it to alert others if you need for help or to alert boat users to your presence in the water if they are too close.
In the case of these Acme whistles, the Day-Glow colours are very bright so you will find it easily if you drop it and this also makes the whistle easily seen by your dive buddy, if they need to find it.