@foxywiththemoxy I don't think they are any rarer than they have been lately, but they have gone down in prevalence over time...specifically with the introduction of the Treasure Chests which are much more common right now.
I found one in a wreck shortly after the specific event came available. However while I was stumbling upon wrecks more often I was not finding any more sunken chests...but as I said this has seem to be an issue since they introduced those treasure chests, making the achievement for turning in the chests harder. However I did find my second one upon sinking a skeleton ship yesterday, so it can still be completed ... you just don't have to follow the literal description to get the sunken chests, as any of the barnacled chests count.
So from my experience the shipwrecks are more or as available as they ever were...I seem to have been finding more of them of late or have gotten better at spotting them from a distance. Sunken chests have been less prevalent (because many were replaced with treasure chests after their introduction to the game), but that has been going on for a while and not due to this current event. Skeleton ships are still very present, even fleets are popping up rather regular so there should be no shortage of ways to complete this challenge. Keep at it.