It must have been in fall of 2009, when some divers reported that they had been addressed by the Coast Guard telling them that on Curacao it was not allowed to shore dive without a permanent dive flag on top of you. When we heard about that at OEW, we contacted the authorities to find out that yes, there is a law on Curacao saying so! Since then we have been recommending the use of dive flags to all our customers (we have some available at Playa Kalki), and use them ourselves on training dives and guided tours outside the marked swimming area.
If you dive on your own, it is totally your responsibility what you choose to do, of course.
Personally, on private dives I always carried an inflatable SMB, so in case I would hear boat sounds towards the end of my dive I could make myself seen before surfacing. The only practical problem you might not solve like that are jetski-riders who occasionally appear at various beaches even out in the far West on weekends and public holidays (there are many on Curacao

) - those you don't hear from under water until they are right on top of you, and they move extremely fast. But those might even use a flag as a turning point, anyway. So just be very careful when you notice they are around. No issues with sailboats or windsurfers, though; they don't use the same beaches as the divers.
Again, if you want to stick to the law you have to tow a flag or buoy on the surface throughout the dive. It is your responsibility to do so, or not.
Greetings from Germany
Andreas