Regarding Specialties. The only "must have" is of course Nitrox.
As others have stated speciality courses are only as good as the instructor.
I chose a Drysuit course, purely because I had no mentors locally who could teach me the basics (Not because I needed the card as I'd just brought a D/S. I found this course invaluable for the basic information, and skills practice. But it was in reality nothing more that a beginners guide to get you started. My point is, Some course can be good if you have no other way of learning from others with that experience.
On the much maligned PADI Boat diver speciality there is an option module
"VII. Basic Seamanship and Small Boat Handling (optional module intended to be personalized for local boating situations)"
The suggested topics (tailored to local conditions are as follows:
A. Local boating laws and regulations
B. Basic rules of the road
C. Basic seamanship.
D. Basic anchoring techniques
E. Basic boating navigation
F. Basic piloting
G. Tides and currents
H. Basic guides to boating safety.
Items D through G in my opinion should be part of the DM course. My point here being stand out as a DM (or as an instructor) you need additional skills, Boats is one, equipment servicing is another. These aren't available as speciality courses, but are one's people should seek out