Dang, that is awful... I'm glad they survived...
I hope the authorities catch the trash responsible for that attack.
Haborless, keep in mind that the flare can bounce off the target and burn through your boat - water will not put it out, only make things worse. Using flares for self defense should be limited to shooting your target before they get into your boat. And you should practice before you rely on it. Spear guns can be used as well.
I think that if one does not wish to use firearms for self defense aboard a yacht, wasp spray is the only viable option. As mentioned, a flare gun is more likely to destroy the boat than an attacker and a spear gun is pretty ineffective in an enclosed space.
On the other hand, wasp spray will shoot a concerted stream around 20 feet and it will disable anyone hit in the face. It is more than painful; it requires professional medical attention, which means the attacker can either go blind or be apprehended by the authorities when getting medical care.
What bothers me the most about these attacks (Bequia, Frigate Island Anchorage, Tobago, just to name a recent few) is that they are happening within tiny communities and yet the police do not seem to be able to identify the attackers, or perhaps they do not wish to?
It is unfathomable to me that the local police do not know the identities of these few violent offenders living in such tiny communities, let alone the persons responsible for the almost daily dinghy thefts throughout the islands.
The safety and security of visitors to some islands has fallen to the business owners and residents (Portsmouth, Dominica and Admiralty Bay, Bequia have established private security groups) as the government officials honestly do not seem to care.
In the seventies, any violence against visitors in general and yachts in particular was not tolerated by the governments of the West Indies, as witnessed by the occasional public hanging for crimes involving violence against outsiders. But I guess the police have to want to catch these criminals before they can be held responsible for their actions?