imho, a camera should meet four basic requirements prior to building an underwater system around it.
1. have a suitable housing available
2. have manual control over aperture
3. have manual control over shutter
4. have manual control over ISO
The S 500 and S 410 meet requirements one and four, but not two and three.
Why is user control over aperture and shutter necessary for underwater use? Simple. These auto cameras are designed to capture images with two feet firmly planted on the ground, shooting through air. Their exposure calibrations do not take into consideration the effects water has on light nor the fact that the user may be attempting to float above a sensitive reef in current. The ability to manually over ride the predetermined "air" settings that the camera will set when faced with these conditions therefore is important. Some of us may be challenged in an attempt to avoid motion blur when our cameras set 1/30 shutter speed on land. What are our chances to remain still enough to capture an image in current?
From my experience in viewing images posted from such cameras, I can predict that in order to compensate for the typical low light conditions found underwater, the camera will open the aperture to it's widest setting and slow the shutter speed down to a minimum. The result will typically be a less than satisfactory background exposure. And if your buoyancy skills are not top notch, as I made mention to previously, expect motion blur.
imho these cameras were designed primarily to fit into a purse or pocket and to offer the user a quality image capturing tool for land use. Building a complex rig (adding external strobes, arms, trays, etc) around them defeats their intended purpose as they grow considerably in size. I believe they would be fine as is in their underwater case for snorkeling where light has not been adversely affected yet by the filtering characteristics of water or for macro use on night dives where background exposures (area of the composition not influenced by the strobe, whether external or internal) are not an issue.
If it were me, I'd upgrade the camera prior to investing in any strobe system that would not be around when you outgrow the underwater capabilities of the two cameras in question.
fwiw,
imho,
my .02psi,
bobf