snorkel for the engine intake (sealed rather well, I watched a rubicon die in 3' of water because the snorkel tube wasn't sealed at the elbow
extend trans breather
extend both axle breathers (this is often overlooked and axles fill w/water)
do'nt get your firewall or dashboard submerged... I don't know how to proof it unless you pull apart and splice/seal every connection.
Also, an ol' mudder trick is to put baby oil into a spray bottle. Before runs, spray the underside of your jeep w/a thin coat of baby oil. Then the mud rinses right off easy...
But yeah, how deep you going? Whats your plan?