^That only happens when you first enter the Lava Field. You see some monkey throw the milk bottle on the rock.
And as for getting the Milk Bottle, a message should be highlighted "use BANANA PICKER on MILK BOTTLE" or something like that. I know you definatly have to steer the boat to the left side of the rock, where a log should be floating there to block your path. If you hit it, it should simply float away.
I don't know, I haven't played the game for a while. Try www.gamefaqs.com
, that oughta solve your problem.