There are pluses and minuses to a water-tight sail tube. The obvious plus is that it keeps water out
, but the minus is that it keeps water in
And water (especially salt water) kept in a tight container in hot weather means a nasty brew of unwelcomed molds.
So, I would recommend simply rolling the sail around the mast. If you use your kayak around sea water, it is always best to rince the sail in fresh water before putting it away for a long time (salt crystals are quite abbrasive). For protection, a PVC tube is great as long as it has holes for drainage and ventilation.
There's no better place than on the water. Happy kayaking and sailing!