Crab Islands is the twenty-second mission in WorldBuilder.
Ignore the western island and the Forklift plans; you do not need to build a Forklift at any point in this mission.
Using your Freighter, grab the plans for the Defender to the west. Be careful when sailing around the islands, as even though Crabs can't traverse water, they can still attack your Freighter from the edges of the islands. Grab either stack of yellow bricks, and drop them on the eastern island. Build a Defender and disassemble the Crab, then grab the plans for the Snail. Take apart the Defender, use the Freighter to carry its parts to the center island, then build another Defender and disassemble the Crab. Use its parts to build a Snail for the goal.
Grab the Marina plans by the eastern island; the same place you found the plans for the Snail.
One way of achieving the Bonus Goal is this: take apart the Snail and the Defender on the center island, and use their parts to build another Freighter. Take apart the Marina, and have both Freighters carry its parts towards the goal. If your Freighters are running low on energy, simply put the parts down and rebuild the Marina to recharge.