Seaweed goes in sea

<p>Three separate bugfixes so that vanilla seaweed goes in sea.</p> <p>One of the bugfixes also applies to mods that add blocktypes that belong in the sea, so is generally useful.</p> <p>Only really makes a difference if you set land cover to less than 100%, i.e. you generate oceans in your world. Does not apply to freshwater spawns.</p>