With regards to using FO with ikelite housings, it's possible with canon DSLRs as you can tirgger the flash even with the flash 'half popped' up.
There are a couple of concerns with using FO with DSLRs. I'm using a Nexus 50D housing which allows me both options and i've tried both.
1) Using the camera Pop-up flash allows you S-TTL/E-TTL with Inon/S&S strobes. However.... You have to take into account the flash re-cycling time on your camera meaning that you're unlikely to fire off multiple shots and you'll be stuck waiting for the camera flash to re-cycle. I think this is especially true shooting macro at high f-stops.
2) Cameras like my 50D doesn't allow you to control the flash output manually, which may be an option to some DSLR users. But if you're manually controlling your camera flash output to save battery, lower re-cycle time etc, then you might as well go the hard wire option and control light output manually on your strobes.