My thoughts here, probably already said by everyone else...

1) You're right.
2) If you're sticking w/your Altoids tin use a lot of electrical tape.
3a) Strive to make everything in your PSK capable of enduring long term water exposure.
3b) I don't often carry a PSK. When I do it is in a round candy tin well electrical taped. A Pelican case or Otter box would provide more water protection (and I'm often on the water) but at the cost of substantially more bulk. Everything I do carry on me, which again usually does not include a PSK, I want to be able to tolerate what I can tolerate. With the exception of my cell phone I've completed this mission. My bic lighter, Skeletool, keys (including an ARC AAA), Surefire E1B, wallet, and lip balm have all been thoroughly soaked in saltwater and after a healthy rinse with some fresh water are no worse for wear.
4) Make sure you roll down the tops on those dry bags. Nice flat folds in the roll. If the bag says you need 3 folds make it 4 or 5. I've had great success with roll top bags but only when I do my part.
5) If you're storing things in waterproof containers for any length of time keep desiccant with said items to the extent feasible.