So the first time I played shipwrecked, around day 40ish I encountered sealnado, he killed me of course, but ever since then he has not spawned once. I have started 7 new saves (games) and done 3-4 full hurricane seasons in each one yet no sealnado, a few of those I even turned hurricane on very long and sealnado on more yet nothing, seems bugged out imo and needs fixed bad, I haven't even got to fight him yet and I would really love to have iron wind.
Steps to Reproduce
Maybe let sealnado kill you on first play through then start new game and see if he spawns again.
This issue has not been confirmed by a developer yet.