So, I didn't notice a blocked storm drain. I had about two inches of water everywhere in the basement. As you can imagine, I'm going through a thorough cleanup down there. As such, I have a few questions regarding lighting equipment that has gotten, let's say, a little wet. Here's what's affected and what I've done :

1. Metal halide lamps in sealed boxes that got quite wet : I put them in front of a fan in a heated room and the boxes dried up completely.
2. F40 preheat NPF ballasts (potted) that got a nice soaking : I wrapped them up lightly in a towel, will likely put them in front of a fan tomorrow. Their boxes went in the dumpster.
3. HID ballasts, capacitors and ignitors : I have them set on newspaper for the time being. Probably put them in front of a fan likely in a heated room, probably tomorrow. There are several non-potted small fluorescent chokes in the mix, too.
4. Dry film capacitors that got wet (a good soaking) : Can they be dried out or should they be discarded ? Same for North American cylindrical ignitors, they appear to be potted in resin in their aluminium case.
5. F40T12 and F18T8 lamps, almost full cases, were leaning against the wall, the bottom got a soaking. I laid them on their side, the bottom is busted out (the lamps are visible) and put a fan to it. Seems to have almost dried up and I plan to glue a new bottom on the cardboard box because I have no other means to store them. I only have 3 cases.
6. Christmas incandescent indoor sets, new in box, got a very good soaking, were discarded because I would never trust them (fear of rust in connectors, I know some companies cheap out on them).
7. A couple of electronic F32T8 ballasts that got a good soaking were discarded.
8. Several used 2xF40T12 HPF ballasts, not discarded yet, but I'll have to inspect further. Some may have leaked when they were used previously so I'll decide later.

Many thanks to any useful information you may contribute here.

Step 1:
Rinse everything that got wet with storm water with CLEAN water. Make sure to open up any device that can trap water, and rinse it out (like fluorescent starters). Get a squirt bottle for the small objects. Then rinse with some demineralized water with about 10% isopropyl alcohol (rubbing alcohol) in it. This is to make sure any debris, scale and salts are properly washed away, and the alcohol makes it easier for the water to penetrate small corners and to evaporate.

It's better to get it wet a 2nd time, than to leave small amounts of  salts, scale and debris in them.

Step 2: put everything on your central heating registers, on a piece of cardboard that you put on top of a central heating radiator etc and let it sit there for at least a week. You want everything to get thoroughly hot and dry.

Step 3: borrow a megger and do an isolation test.

Again, i really want to stress the importance of rinsing stuff with CLEAN water. Water is pretty much harmless to electronics as long as they're not turned on and properly dried out after washing. But tiny amounts of scale, salt, debris and earth can cause leakage currents and trap moisture.

Of course do not megger test wet items! Also megger testing electronic ballasts is not a wise idea too. Anything potted will certainly not be damaged with a flood, just rinse the dirt from external connectors.
 

Depends on your test voltage. The filter caps used in mains to earth filtering are supposed to be able to handle peak voltages of 2kV or 4kV or thereabouts. That's why they have that Y2 marking on them.

You don't have to test at that voltage, 500v is plenty for a leakage due to trapped moisture or corrosion test. It will, of course, also pick up leaky paper capacitors if those are present.

You probably agree there is not much sense to megger test electronics. For open types, it is more practical to look at the board condition, and clean up the dirt as necessary. For potted, there is generally not much risk of insulation degrading from brief contact with water.

Megger testing makes more sense with electromagnetic things - where you can expect some inner layers of insulation get soaked with water. Especially with older types with paper insulation. Less for modern plastic film type insulation, which is typically not hygroscopic.

That reminds me an old story from my school years, when I found a nice 220/36V safety transformer of about 150W in a flooded control box on the construction site nearby. I attempted to dry it, but likely not enough, it ran for some time, but then shorted with some sparks. I almost cried of disappointment - these were hungry Soviet years, and such transformer was quite a valuable asset then... Not like today when you can just get another one easily from a second hand listing.