YS-D2 full flash duration - 4.2 ms. You can use full energy at shutter speed slower than 1/250.
YS-D250 full flash duration - 23 ms. You can use full flash energy at shutter speed slower than 1/50.
But how often do you need full energy? - very rare. In many cases we use only a small part of it, and there is no problem to set more fast shutter speeds (up to Nikon's max synch speed 1/250).
Sorry, I don't know Seaflash-60 flash duration. There is a round tube inside, but tube's diameter and thickness are small. I can suppose duration about 5...8 ms.
Underwater objects move rather slowly because of the water, flash freezes them fine, independently of strobe type and duration. There is no need to specifically take care of this.
Not all underwater objects move slowly as can be seen by the blurred critters in this photo taken on a night dive with an Inon Z-240 (shutter at 1/125):