I just recently took a new pair of YS250Pros underwater - fortunately checked them both in the pool prior to diving. One of the battery caps had a leak - similar place to others in this thread.
On my battery cap there is this little rotating gizmo inside the cap that has to align with the battery housing in order to thread on the cap - my suspicion is that I misaligned this at some point while playing with the strobes on land after delivery - it looks like it would be fairly easy to crack a cap if you threaded it on with it being misaligned - the back of the cap would receive the full pressure of you tightening the cap with the "gizmo" caught between the housing and the back of the cap.
This "guide" inside the cap rotates freely, so when you are threading the cap its a 3 step process
#1 align the rotating guide so that the mark inside the cap aligns with the cap
#2 align the cap on the housing so that the mark on the cap lines up with that on the housing
#3 try to thread this mess on without anything moving
Any ideas what the function is of the guide mechanism inside the cap is? It appears to have a cutout opposite the guide marks to allow the battery cable to protrude into the cap - but otherwise doesnt seem to have a function - but hey, what do i know
The rotating guide is only used to fix the battery pack, in my opinion.
I think many owners have problems with leaks in the battery compartment in the strobes.
My experience with silicone o-rings in dynamic use is not good. I have also Inon, wich have also silicone o-ring, and i think they to fragile and soft for that kind of use.
When they are not well greased i think it is possible that the sealing thickness is not the same at each place.
So, all my dynamic o-ring seals are replaced by NBR-70° o-rings.