If you can open the back of your watch, then you can seal it. Here is a tip on what grease you can get anywhere. LipBalm. In stick form. Yes it works. Not exactly perfect like a proper grease, but hell it works very very well. And does not harden or soften in a broad range of temperatures. And clings good on the silicon seals and stainless parts. And safe for any rubber, natural or synthetic. Many jewelers use it. They never ever show you. But when you are gone, bam ! They look left and right, and they wip it out from a hidden compartment and rub-a-dub the stuff to the watch seals and threads. You can also grease-up the crown 'O' ring with a childs little soft toothbrush 'poked' into the lip balm stick. If you are handy, you can do the crown stem to by pulling it out on time setting mode. Put a lot on the stem, the inner crevice or hollow of the crown by using your tiny toothbrush, so when you push it back in, it will grease up the inner mini 'O' ring inside the crown tube wall. No grease will get into the watch by the greased up crown stem. The inner mini 'O' ring prevents that . Always make sure that the caseback is nice and tight after that. Snug is not the word. Tight is the way.......Don't worry, if its good and safe for delicate lips, hell its safe for your Sub and 'O' rings !! ............. In regards to not knowing if your crystal is set down all the way tightly, well here is another tip. And no, you wont break your watch, crystal or anything. Get two thick telephone books, lay the first one on your floor, lay your Sub or PO down face up on the first phone book. [remove the bracelet first] then lay the second phone book on top of the watch. [best to use phone books with ripped out cardboard covers][you want a soft but stiff surface to spread the downward pressure while you sit on it] Now slowly squat down and sit on the phone books. wait a few seconds. Don't twist. Just sit. Make sure to center your butt smack in the middle. Now lift up your butt slowly upwards and remove the first phone book. You are done. And safe too !! No harm done, even pressure exerted all over, and no over leveraging to worry about. Slow and easy does it.