Southern Italy better in mid May or Early June?

Late May is good all around, but for swimming in warm seas, I would lean toward starting closer to Memorial Day.
I travel around this time every year, and the sea is still pretty chilly for me, while crowds are never an issue (except in the obvious places).
I would google the ferry schedules for the Aeolian islands, but on the Amalfi coast schedules will be fine.

I think you have a bit of a trade-off: With water temperatures that are really comfortable for swimming will probably come land temperatures that are hotter than you would prefer. Sicily was quite hot from June 16 to July 4, 2015. I went from there to Puglia, which seemed worse--which I assume was a reflection of the calendar rather than the geography.

I am not a beach person so cannot comment on what swimming would have been like in mid-June.

I might try to split it, say from the 23rd-6th. Not long after that schools will be out and vacations will start.

Two nice stops traveling up the coast are Palinuro and Santa Maria Castelabate.

Thanks all.
Yeah I think a middle ground probably works best to avoid crowds but allow for warmer waters.

Really swimming the ocean is not a big deal for me and not planning any lay on the beach relax days just want it warm enough that if the hotel or rental property has a pool that it would be open and swimmable at the time we visit. Not everywhere would have a pool but maybe a couple out of 5 stops could.

So many gems on this lesser visited half of Italy:
Palinuro and Santa Maria Castelabate do look really nice and close to Paestum a place I would like to go but seems hard to fit into the trip so need to research more. Also I know Maratea along the coast we would like but now are not planning on visiting as it stands now either.

We really are only touching the Eastern part of Sicily in my plans.
After Siciliy stopping in Tropea and I was planning on moving a bit east to get to Matera next before cutting back west to either Castelmezzano or Pietrapertosa before the Amalfi Coast town. (prefer Positano and have been before but since plan on driving will probably settle for a less expensive town along the coastal road area there and rent something rather than hotel)

From our base in Matera (figuring 3 nights in one of the caves/sassi hotels) we could have a full daytrip drive to see Puglia for some unique sights like Alberello and Polignano A Mare.

Subject to change at this point.
Lesser photographed beautiful places are definitely one of the priorities for me.

While in Matera, we took a fascinating tour of Craco, an abandoned village.
I would try to squeeze in Paestum, just a stopover would do it, but you are correct, you could spend months on that route and never see it all, enjoy!

Assuming you want to keep Memorial Day within your vacation period, you have a better chance at finding warmer waters later rather than sooner. I’ve been to the Amalfi coast around the end of May and the water was swimmable but kind of on the chilly side. Not like here in SF where even in summer you need a wet suit, but certainly not warm either. Same for Sicily. I spent days on various beaches at the end of May, and although there were people on the beach sunbathing, few ventured inside the water.
Going a week or two later may not make much of a difference, but even a degree helps.

In terms of crowds, I don’t think it makes a lot of difference. Many beach towns in Sicily were ghost towns at the end of May. Only Mondello or Giardini Naxos on Sundays had several beach goers (but nearly none inside the water). Outside of weekends even Mondello and Giardini were pretty slow too. In most places my wife and I were the only fools in the water (plus some Scandinavian tourist for which anything not frozen solid is warm enough to swim in). Even if you go in June, you won’t find significantly bigger crowds. Any beach before July won’t be crowded with the possible exception of Sunday.

Val: Craco looks really cool. Definitely work checking out if in the area. Put that town in Tuscany and everyone would know of it.

Roberto: thank you! I think if I go from South to North and go in the middle time frame wise I will be best off for crowds and weather. Ideally getting to Amalfi Coast are when it warms up enough but before it gets overrun with crowds.