Steve,
Interesting reading...I have had 2 YS250s leak, both times the strobes were still firing and there was only a couple of cm of water in the battery compartment. In both cases the strobe was working one day and leaking another.
Sea & Sea via their Aussie distributor replaced them both times...no explanation on what the issue was forthcoming even though I asked. Pretty sure the cap was not cracked like the one in your post.
Great strobes though!
Cheers Dave Y