Boats: Raise collisionbox top surface to fix boat behaviour

Lowering the top surface to be level with the boat top somehow
causes the boat to fall through world if underwater. Revert to
previous position that is needed for correct behaviour

mods/boats/init.lua

@@ -34,7 +34,9 @@ end
 
 local boat = {
 	physical = true,
-	collisionbox = {-0.5, -0.35, -0.5, 0.5, 0.15, 0.5},
+	-- Warning: Do not change the position of the collisionbox top surface,
+	-- lowering it causes the boat to fall through the world if underwater
+	collisionbox = {-0.5, -0.35, -0.5, 0.5, 0.3, 0.5},
 	visual = "mesh",
 	mesh = "boats_boat.obj",
 	textures = {"default_wood.png"},