I've used on hatches for years with great results. It stays sticky-gooey for what might be decades. I had chronic leaks at some chainplate covers that needed attention every year. The butyl is going on 5 years without a drop. It would be good for the water tank but not if you are going to remove the cover very often. Butyl will pull apart like uncured caulking. For the water tank I would use a foam type neoprene. Check out McMaster.com. They have lots of different material and go good chart showing how to gauge the "Shore Hardness". This link is to foam but they also have solid sheets.

http://www.mcmaster.com/#neoprene-foam-sheets-and-neoprene-rubber-mesh-sheets/=c1piy5