First, 300d or D100/S2. Second, if D100/S2 then D100 or S2.
Does the Nikon glass lineup justify choosing D100/S2?
Does the D100/S2 have features you will need that will be misssing on the 300d?
Is it worth the $600-$800 U.S. price difference?
As for the D100 or S2 debate, we've beat that horse before a time or two.
If I had it to do over and didn't have Nikon glass, I would opt for the D100 or S2. There are features missing from the 300D that I need, but that's me.
As for the D100 or S2, I'm still not sure. I've got a battery pack grip for the D100 with L bracket. I've got other accessories that may or may not work with the S2. I rely on the histogram for exposure evaluation. Using an LCD sceen is wholly unreliable. The S2 histogram implementaion is poor. TTL with the Fuji is a big plus. The in-camera interpolation is a small plus. Having ISO 100 is a small plus.
By the way, the Aquatica 300d housing is based on the A90 housing. If you go back to my review of the AD100 housing, you will see them side by side. The A90 housing is much smaller.
By the way, about flash sync speed. The problem with the D100 is when you have a highly lit background and you want to use fill flash for foreground shaded area, it requires you to stop down, sometimes way too far. The Fuji has a slight advantage in that the ISO can go down a full stop, whereas the flash sync is only a half a stop worse, so the S2 has a half stop advantage even though it sync limit is 1/125 vs. the 1/180 of the D100.
I think you should get the Canon. That way you can be the early adopter. Someone has to go first. That way you can figure out the hard way what the various extensions are needed for all the Canon lenses. Other readers of the board will then thank you.