First, I'm glad you can walk away from the crash and lean a (pretty important) lesson from it.
Second, when it comes to riding you always want to give yourself "an out". This is especially important during this time of the year.
3 things that work for me:
1) Stay back far enough from the car in front to ensure that I can react to things that may be under it. This would include roadwork covers, but I remember coming over the Oak street bridge a few weeks ago to see a football sized rock on the road. I avoided it. Could have been ugly if I hadn't.
2) Once you've given yourself some space and have time to react, look for a space you can ride the bike into if you can't avoid the obstacle safely by braking. This could be between the in front and the parked cars, between the lanes (on a multi-lane road) etc.
3) Those steel covers, while not the slipperiest things on the road in the rain, are bloody close to it. I know that on Broadway just east of Commercial the entire westbound lane is covered by them and has been for a while. If you take a route often, you'll learn where the danger spots are and know to look out for them. Other things that can be amazingly slick are some of the "rubber mat" type covers they put on railway crossings.
You already have the right strategy when it comes to the bike: Relax now, make sure you're fine, the thing (it is only a thing!) can be fixed later.
P.S.: most good riding books have sections on riding in the rain, I know Sportriding techniques does, it might be worth picking up a copy.