Scubbie,
A futrher thought on your problem. You can check whether it is the converter causing your problem by trying without the converter. The YS110a strobe can be plugged directly into the port next to the converter port on the housing. Don't attach the converter at the same time or it may try and take control of the camera setting.
The strobe will work fine in the manual configuration, it may take a few shots to get the camera fstop and strobe power correct (I would probably try speed setting of 1/250 first with the ISO at 200). The manual exposure calculation table from the YS110a Manual gives a bit of initial guidance. For ISO 200 this gives f16 at 1m as a starting point with strobe at full power, you will probably find that you need a stop less, depending on water clarity.
On the strobe you need to set to either Manual 1 or 2 with Slave off.
A bit of messing around but it might resolve whether it is the converter causing your problem.
Cheers
Ken