No clue what they are, but they are incredibly rare. I'd assume that an account with more then one would obviously be worth more.
I have never heard of it and I have a lot of experience in runescape :L
Ive seen a couple of sales in the past of accounts with warships. They aren't worth alot. Around $10-20.
I have read that accounts with Warships are rare yes but hold little value. This is because the Warship cannot be displayed (IE worn, or emot done) to other players besides through screenshot. Players don't like to just have rare items, they like to be able to show them off its old, its cool but it has no flash like the sythe or bunny ears do.
Disk of returning? Pumpkins? Easters? Lol.
Those 3 are tradeable, and you forgot the Half Wine.
Warship is not tradeable.

