I usually carry a film body with me for land-shooting and trips and keep a strap on that. If I want to shoot topside with the digital, I just do without a strap, as it takes too long to attach and detach. I have a small Lowepro holster case that fits around the S2 or F100, and it has a strap.
Underwater, if you're using one of the larger housed DSLRs with big strobes and long arms, the retractor systems with plastic clips designed for smaller cameras may not be the most appropriate.
I have a pair of brass "suicide clips" attached to the strobe arms near the clamps between the two arm sections. When the arms are folded and clipped in, both clips are at the top. I clip one (and sometimes both) to the chest D-rings.
This, by the way, is the only place I'll accept suicide clips on my gear. I used to use bolt snaps, but sometimes found it difficult to clip on or unclip at the surface in heavy waves, either when I'd just been handed the camera from the boat or wanted to hand it up. The suicide clips are attached to the arms with large stainless steel rings, which can be removed if the clip somehow got jammed or fouled, which seems unlikely.
Using two clips makes it easy to quickly find one and clip it on. It also makes it possible to carry the rig on my chest easily and symmetrically for long swims against a current, or if I need both hands to set a delayed SMB or to render assistance to another diver.
If I've got a long walk with the housing, I sometimes clip both of the suicide clips onto a spare arm section or something similar and use that as a handle for balanced carrying.