I have tried lots of different solutions for keeping maps and charts dry.  Most haven't been very successful.  I have tried a number of map cases and the problem I've found is not that they don't keep the water out but that over time, usually in the course of a single trip, the plastic seams tear.  Last year I picked up the Sealine case, the one with the roll down top, and was very pleased.  It did what it was supposed to do.  That said I've only used it for one season (about 30 days on the water) but so far its held together and kept the water out.