All handrails are straight when purchased. You curve them as you install them, starting at one end and working to the other. By cabin rails do you mean the handrails inside the cabin? Through bolting with washers and nuts inside id best if you're going to depend on them. If your cabintop is cored make sure you pot the holes by overdrilling and filling with thickened epoxy before redrilling the proper size holes for the fasteners. This link will help. Sealing Deck Penetrations to Prevent Core Rot Photo Gallery by Compass Marine at pbase.com
If you do it this way there should not be a problem with leaking.