for pencils they sell all graphite ones in art stores, basically the whole pencil is made out of the "lead" part. the work great and water is never an issue.
wrist slates are not a problem if they are the compact ones with 3 flipping pages. change the mount to a bungee and they are easy to slip on. they are very useful to keep your runtime on, that way they are next to one of your computers.
For useless: Spare air, housing+software to turn your iphone in a trimix dive computer, integrated inflator/2nd stage, mask with uw camera (max depth 40ft), colossal snorkels.