If you don't need access then 2-part expoxy putty is easy to work with and fills space nicely (like Selleys Aqua Knead-it). JB Weld and Devcon are more liquidy and will seep in as they cure but are nice and strong.
If you only need rare access then I use polyeurathane sealants like Sikaflex Marine. it expands as it cures.
If you need occasional access then I use liquid gasket sealant like Silastic (silicone). It doesn't expand as much as polyeurathane but is easier to crack open.
If you need access after diving like the battery recharge port then an O ring is the go.
A good metaphor for your torch is likening it to a submarine. Compared to ships submarines have much shorter lifespans as the container compresses and relaxes with every dive. Compound that is unequal heating expansion throughout due to the high intensity lighting systems we use, plus different materials in contact with each other and this beomes a recipe for wear and tear, and stress cracking.
Bottom line, limit the accessable ports to only that which is necessary.
Klem
