Today it's really just destroy any crypto you have - defined as keys.
Burn/shred/destroy any books, tapes, or other media. For the boats, if you can't do anything else, at least toss them overboard, preferably in a weighted net bag or something(toss something heavy in there!) so that they don't pop up immediately.
As navy guys should be able to tell you, in any depth of water finding such a thing is a huge pain in the butt, even if they 'know' where it was dropped.
Zero any key storage devices such as the CYZ or SKL . Zero any KGs and other commo units with active keys in them. Remove the keys and toss them (overboard). If you have time, break them, but don't worry overmuch.