It depends how old the shipwreck is what you may see. You could find coins, cannons, timbers, or crates on an older shipwreck (Assuming it hasn't been totally destroyed)
On a newer shipwreck you could find rusted metal, rusted guns, machine parts, old cargo.
If you find any identification numbers then you should turn those in to the local authorities to see if the ship/plane has been reported yet. (You could solve a mystery and give peace to someone's family)
Regardless what you find you must leave it there undisturbed. Salvage laws are VERY strict and you could end up in prison for a VERY long time if you take even the smallest thing.