I think there is a few other options than auto decay. In another post I can't find right now, but I found it after my own ship sank, someone said that the trolls are going around over burdening ships to cause them to sink. Also I noticed a lot of damage on my new ship and i repaired everything I could, and checked again today, and more damage = some weird ass game mechanic just killing ships for no aparent reason (my ship rn is just anchored near an island, and i haven't seen any sharks or anything hitting it so...
Further details: I'm on pve so I'd assume nothing like enemy players sailing up and unloading cannon fire is causing this.
P.s. Uhh is there offline damage protection like in ark?
P.S.s. ERhmm I should probably mention to op that I am also new to this.