Froggy over at Froggy Ruminations: Worried About Enemy SCUBA Divers? suggests that those who have spread alarm over enemy SCUBA divers are overreacting. As one of the accused (see this post and this one), I understand Matt's argument. As a diver myself, I doubt if civilian SCUBA training teaches sufficient underwater navigation to allow for much in the way of night operations or bad visibility (a lot of the water in the world's ports is not as clear as that in, say, Guam). And, yes, there are significant weight and distance issues that hamper diver attacks.
But Matt's argument mostly applies to US Navy ships. There are a lot of other ships in the world and not all of them are as well guarded as Matt describes. And divers need not always be interested in blowing things up. And there are a many ways in which a ship can be used to accomplish terrorist ends other than its destruction. And, in addition to ships, there are a lot of other targets out there that would be within the reach of even moderately capable divers.
So, sorry Froggy, I still think there is the need for some concern.
Update: Added the word "think" to the final sentence. Preview is my friend.