I love otter boxes and pellican cases For first aid kits in boats, the one I use has extra room so I can stash my wallet and cell phone in it. its also not bad for a first aid kit for an extended backpacking trip where you might hike for a day in the rain. or on a bycycle or motorcycle trip where rain may soak saddle bags.
For a PSK it definitley seems a bit bulky, as has been said, most psk supplies (trashbag emergency blanket chord, ferro rod etc.) are not really that water sensitive. if something is you can get a smaller waterproof case like a match case, pill fob, or ziplock bag.
Waterproof bags really do work well, walmart has both the nylon "barrel" type ones and the ones that are like ziplocks on steroids. these are nice since you can put them in a pocket. the plastic ones have a lanyard so you can put in in a pocket and lash it to your belt or PFD for extra security. Just check them for leaks before you go out,they are pretty tough but a pin hole will allow water in and now you have first aid kit stew.