as glasman said silicone or tying them together (preferably with fishing line) but you could also screw then together, and there are safe epoxies for tanks you just need to let them totally cure first.
some type of glue is really the only option due to the orientation and shape of the pieces of wood. I was thinking of trying silicone but I wasn't sure if it'd stick to wood real well.
Super glue stablized with fishing line is my suggestion as well- I read it somewhere and I agree- if reefers trust it in their tanks, I'll trust it in mine, and it cures in water- they use it to secure frags- get the gel cyanoacrylate type though. I'd stay away from screws, unless they were completely non-reactive, as lot of coatings on screws can be toxic. Maybe Stainless Steel would be alright.