Quite prevalent. Mostly in Asia, where the straits of Indonesia are plagued by fast boats armed with automatic weapons and RPGs, attacking both big cargo vessels, coastal traffic, and yachts. I hear the Horn of Africa is pretty bad too, the boats originating in those lawless non-states there, but the US and European navies have started patrolling there with some limited success. The Caribbean used to be pretty bad a few years back, mostly drug smugglers hijacking yachts for one-shot deliveries. The crews, of course, were killed. But the War on Drugs has cut down on a lot of that activity.

Pirates never fit the stereotypes of the movies. They usually have small, fast boats that can outrun warships, and operate in areas where there are many confusing boundaries and borders, poor local enforcement, lots of shallow water, hidden harbors and narrow straits that funnel their prey to them.

Ships and yachts avoid them by staying well offshore out of their range, if possible, or by forming convoys. Major navies usually don't get involved unless it threatens their trade.