likely pretty rare, I have one that has the main 5 all contiguous, and from either end you could swim a narrow channel to reach either of the other two. You can walk to all the bosses and the trader from world spawn.
At 3 and 9 o'clock on the map, in the seed reader just hit random a few dozen times. You may find that and others that are intriguing. It is how I found my Mtn Ring Trader/Citadel Seed.