The Canon S100 is an excellent point and shoot. However, it has a very small sensor. Small sensors limit the resolution you can achieve. Small sensors also limit high ISO performance. However, it depnds on what you are using the camera for. If you want excellent performance over a range of performance issues like macro photography and wide angle photography, you will want a larger sensor. By excellent performance, I mean that you intend to print out large enlargments of your photos so you want a high resolution.
Now if you want a camera that is compact, light, and capable of nice snapshots and crisp photos of smaller enlargements (8X10, 11X14 or maybe larger) then the S100 should be fine. A good photographer can get excellent photos with a point and shoot. Remember the quality of the photography is more about the photographer than the camera.
The mirrorless cameras have sensors at nearly the same size of the DSLR cameras. They also have a range of lenses so they can handle wide angle and macro photography. They cost more than the point and shoots. But they are quite compact.
If you are not using RAW or using strobes, getting more than a point and shoot would seem to be over kill to me. Now some people shoot with existing light and play with the effects. Some of these people really can use a DSLR effectively but it takes quite a bit of knowledge to pull off.
I have a DSLR and it and the ports and other gear required for it weighs as much as my dive gear. It has the advantage over the mirrorless cameras in having a very fast and precise focus.
You might want to get Martin Edge's book on underwater photography. It is not cheap but it will give you some appreciation for more than the basics. Even if you are buying the Canon point and shoot, you are talking about around $1000 if you are talking about strobes, housing, camera and wet lenses. So the more informed decision you can make the better.