Originally posted by electricdawn:

Well, that's the you are being hunted thingy... They will get you everywhere.... well, when they can reach you. ;)

No, they wont. He was sailing near the small islands in plains biome. That is where he triggered the raid. It is not possible to trigger any raid in the ocean. Well, at least for now :)
@OP, islands are the safest location to make a base. Just make a few workbenches or campfires around to prevent mobs from spawning and you will be completely safe. If it is not far enough from the mainland, drop a few campfires on the shore too. You will get the red circle and event message, but no mobs will be able to spawn.
And yes, trolls and other mobs can spawn outside their biomes during the raid.