Best story EVER!
I second the advice that what they're doing is perfectly normal. What did you expect?
I vote against the "screw your players more" advice that has been given. Just make sure several letters in a row don't blow up on an NPC(or whoever). Unless you have been doing other untoward things to them, they will probably eventually decide to start reading themselves again eventually. At which point you should not immediately lace another note with explosive runes. Why is there so much DM-player antagonism?
I thought this might be about water. Are your players afraid of water?