Plane or Ferry Boat Mid May

The seas are generally not very rough in May, but you can minimize the risk of seasickness by taking the larger, more stable conventional ferry instead of one of the highspeeds. Blue Star Ferries runs this route every day: https://www.bluestarferries.com/en-gb

how early do you need to arrive at the airport or port when traveling
between islands?

For domestic flights, at Athens and Santorini airports plan to arrive 1h to 1h30 before the flight.
Note that with Aegean/Olympic or Sky Express, using their apps, you can check in 48 hours in advance and get your boarding pass on your smartphone

About the ferries, plan to be at least 30 minutes at the port before the arrival time of the ferry.
Anyway they are rarely on time except early in the morning when the ferry starts its journey. In this case you can be at the port 1 hour before and board the ferry.

Don't worry too much about the ferries and as Lee tells you, favor the traditional ferries (Blue Star Ferries type) with which you feel less confined than with the fast catamarans like the Seajets.

When I go island hopping in Greece I always watch the weather forecast. In particular wind forecasts on:

https://meteo.gr/index-en.cfm

(click on Greece, Cyclades and choose the island)

From 7 or 8 Bf wind force with strong gusts on the day you have to take the ferry, there is a risk of cancellation or severe delay.
Above 8 Bf it will most likely be a sailing ban for all ferries, even the traditional ones

Thank you so much!!

I have taken the Blue Star ferries in Midmay at least 10 times ... from Naxos to Piraeus and Naxos-Santorini. If any Month is smooth sailing, it has to be MAy & early june ... Those Famous Meltemi winds are usually August onward... in Mid-may, in my experience, the sea is like glass ... you have less experience of motion than on a train. Sometimes the ice doesn't even tinkle in one's glass.

Frankly, unless you have quite a lengthy Greece stay... 3 weeks?... I find that long Blue Star ferry trip ( PIraeus- Naxos or return) takes too much of a day -- nearly 6 hours. For that length of time, you'd want a "reserved economy" ticket. also called "aircraft seating" (although it's nothing like todays awful cramped Economy torture-seats in big jets! these seats have legroom-upholstery-leanback like jets' 1st class). That's nice -- but "reserved-economy" price is not that different from $$ for. domestic flight on Aegean or Sky Express (If u buy on right dates). And ATH-NAX is 40 mins vs 6+ hours! Therefore, consider limiting your Blue Star experience from Naxos to Santorini..for only 2 hrs -- so u'd only need standard "basic economy" seats..

Of course, I also advise that the girls become Speed-Sisters, going on board. Why? Because the best seats -- open rear-decks on the 2 top "economy levels" -- are 1st come/1st served, and by Naxos, will be very full ... except for those getting off at Paros & Naxos. The strategy is to be on the pier VERY EARLY so you can be at head of waiting line. As soon as pier guards open the gate, people will sprint for the ramp, drop their heavy bags at the vehicle level (perfectly safe, everybody does it), and hustle up 1 flight of escalator, then steps. Thus, it's best that YOU wrangle the bags while at least one of the girls races up to find 3 seats... then she can wave at you when you arrive a few minutes later.