They dock the Spire at the Radio Tower (basically a radio tower in the middle of the ocean, surrounded by some wrecked ships tied together, like a little island town in Waterworld if you can imagine) to get some supplies and find a diver.
There he gets hooked up with a diver named MAY PETROSKI. (The scene where Sam announces he needs a diver made me LOL.) As they are heading back to the Spire and haggling a bit over money, someone starts shooting a cannon at the Spire. Then Capital Sam is trying to figure out who would do such a thing.