For the boat: http://forums.bukkit.org/threads/mech-pickboat-v0-4-change-boat-destruction-behavior-1-1.2628/ But I dunno if its updated...
IMO, this is a bad idea. Take this example: Player1has a lot of items he does not want, so he drops them in the ocean so they will be hidden from other players until they despawn. Little does Player1 know that items do not despawn, so he is dumping his items constantly with the idea that they disappear. Player2, Player3, and Player4 are also doing the same thing in nearby areas. The amount of items that could possibly build up from this could make parts of the map laggy and unplayable. It would also put a lot of stress on the server to keep track of the items and to keep them spawned.
Maybe, you could make a plugin that lets items stay "spawned" for, say, 10 minutes. After this, the server could broadcast a message saying ITEMS WILL DESPAWN IN 1 MINUTE Also, maybe in the same plugin, could you make a way to select an area and make it so mobs don't spawn naturally or despawn? I'm making an adventure server and I need something like this. Thanks!
I might make this if i got time,So if someone else is making this then plz tell meBut i am still going to post this on the bukkit page.