I was changing a switch junction box today, and noticed something. The wiring is old and there is no ground wire. Above the switch at the joists is plumbing work, including a few sweat elbows and a feed with a shut off valve for the dishwasher. I would like to use some kind of waterproof conduit, box and switch cover as a precaution, and would like advice from the experts, As a precaution against leaks which would likely be right above the switch. I looked at pvc which is very good but not sure how I can secure the box. Thanks.

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If your using conduit of any type, use a weatherproof box
with a closed switch plate....



These are for outdoor work, and are seldom used just because
the box is "near pipes" in a home.

What are the odds you're gonna have a leak right over the box??

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There are rules about piping above panels (in working space section) but nothing about piping above wiring or devices. There are lots of places where this will be found. I agree there will be a very slim chance of a copper fitting leaking. You said you have a dishwasher valve there too, that may have a higher chance of that leaking. If you see its wet don't touch the switch or any wiring around it.
If you are worried about it use outdoor PVC conduit and boxes. Don't use metal since you don't have a ground. Use the switch cover that was posted. At the ends of the PVC conduit use a fitting to prevent cable damage. If the wiring is so old that wire insulation crumbles when moved then as you probably know should be replaced.

I looked on the net about this and found that If your old wiring is bx cable, it may very well be grounded by virtue of the metal sheathing. The metal switch box may be providing the grounding you want, that a PVC box wouldn't.

Since this is in a kitchen, you should install a GFCI in the circuit. You can do this even if you find there is no ground. It's pretty important in potentially wet areas.

I think it might be worthwhile to have an electrician do this and make your kitchen as safe as you'd like.

Good point about the BX. If the wiring is BX then the boxes must be metal. Plastic wound interrupt the grounding continuity.
A GFCI regardless of grounding can be installed on the first box in the line and it can be wired to protect other downstream receptacles, which can even be 3 prong with a label stating "no equipment ground" if no grounding is present.