If you stick to the islands close to Santorini, Naxos and Paros, you can reach them on a Blue Star Co. very large and stable car ferry. Seasickness won't be an issue. These islands have lots to offer and both have excellent beaches.

I have been using Greek ferries for decades to scores of islands, I suffer from seasickness very badly. The best way is to avoid travelling during July/August period at all.
The Meltemi wind peaks then making it the worst time of year for ferry travel in the Cyclades islands.

The avoidance measures I take are to travel in May/June (lighter winds on average) and delay a departure from an island until it is calmer weather. Always enquire about the ship, the bigger the better.

You can also go to a group of islands like the North Sporades (Skiathos, Skopelos, Alonissos) which are outside the Meltemi wind area, also short ferry/hydrofoil connections. Superb beaches (Santorini's beaches are rubbish).

Tablets work for some people but not everyone unfortunately.

Santorini has very good flight connections, both with Olympic and Aegean airlines, so seasickness need not be an issue. Your budget would stretch further if you limit the time spent on Santorini, and visit another island for part of the trip. Naxos has good beaches, and is on the Blue Star route mentioned by brotherleelove.

The Cyclades are the island group most affected by meltemi winds, which are strongest in July and August. Wind is less of an issue in the Ionians (Corfu, Kefalonia, etc) or the Sporades (Skiathos, Skopelos, Alonissos).

i wasnt at all concerned but now people are making me very nervous about the winds in august .i recently got very seasick on a small-ish boat in the great barrier reef (bad weather--rough water). ive been on large ferries and cruise ships before without a problem. we were planning on taking a ferry (the large car carrier) from athens to naxos and naxos to santorini. should i be really concerned?

Chances are small that you'd be affected. The Blue Star Naxos and Blue Star Paros are very large and modern ferries and they're equipped with stabilizers to make the trip smoother.

No, don't be concerned. I have been on the Great Barrier Reef and Whitsundays, and experienced far rougher water than ever in the Aegean. The meltemi usually blows for a few days, then calms down. You may never even experience it.

But if you are really that worried, why not just fly? I'd rather spend 45 min on an airplane than several hours on a ferry any day.

like i said, i wasnt ever worried. we considered flying but then figured-wouldnt it be beautiful to see the islands by boat. plus-i didnt think flying to naxos was an option. im not sure what to do now. how long are the boat rides from athens to naxos and naxos to santorini?