Boat with Everything

Adds some creative-only utilities to fix boats in place, intended to be used at the Modfest event. Source code contained within the modfest branch on Github.

In creative mode, right click a boat with:

• a piece of flint to fix the boat's position in place and make it undriveable
• a golden apple to toggle the vanilla Invulnerable flag
• an iron ingot to toggle the boat with everything "locked" flag, which prevents taking things out of the boat (now that i think about it the invulnerable flag does this too)

You already can't take books out of a lectern in adventure mode