Here's a small explanation from otherwiki's discussion page of what my bro theorized:
On the first boatride I took today, I saw Dredvodd's shadow spawn behind me at the start of the boat ride. However, I moved to the white square across from the tonberry NPC and stayed there the entire ride, Dredvodd did not pop. For the next boat ride I had my brother join me, again I saw Dredvodd's shadow spawn behind me at the start of the ride. My brother stayed at the start and I moved to the white box again. Dredvodd popped after the "Two currents" speech from the NPC, and he spawned where his shadow had appeared and where by brother was standing. On previous occasions, I have NOT seen Dredvodd's shadow behind me at the start of the ride, and when I stand in the white box, he'll spawn behind that. I believe this shows that Dredvodd can "load" either on the "right" of the boat (where the white box is) or the "left" of the boat (right behind where you start), and that he will "spawn" the next time someone is close to where he loaded at certain checkpoints. For example, every time he spawned on the right side of the boat, it was after the "fished up" message, and all the reports i've read of him spawning on the left side is after the "Two currents" message. This also means that it's impossible for him to spawn when no one is on the boat, and that he'll just stay "loaded" on the boat until someone aggro's him and makes him pop.
As for the repop time, I still have no clue. I dont think it's a 21-24hrs, I camped for 12-18hrs every day and usually only saw him about every 5 days, however that may be because I always stand in the same spot, and was standing on the other side where he "loaded". It's also worth noting I have killed him twice inside the same conquest tally. --Jawat
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Given the above, it might be prudent to run around the perimeter of the boat like a lunatic to "aggro" Dredvodd from under the boat. Though maybe there is only 1 second during the ride that he will aggro/spawn on top of the boat, and before/after that, it doesn't matter how close you are. In that case, running wouldn't really help.
The shadow he mentions comes from reports of being able to see a shadow at the beginning of some boat rides for a split second, and that this shadow is actually Stubborn Dredvodd being directly below the boat's deck. Obviously, the shadow doesn't show up all the time; it only appears when Dredvodd has "spawned"/loaded underneath.
Also, keep in mind that this theory was created just after we finally got the ring to drop, so we didn't actually get to trying to test/debunk it afterwards. We had enough of this NM
