Aegina is not a large island — you can drive around it in well under two hours — but its coastline is more varied than its size suggests. The sandy beaches at Agia Marina and Marathon attract the bulk of summer visitors, complete with sun loungers, water sports concessions, and beach bars. These have their pleasures. But the island also has a scattering of rocky coves, small pebble bays, and flat limestone shelves where the water is just as clear and the company considerably thinner.
The southern coast between Perdika and the uninhabited islets offers some of the best swimming away from the crowds. The water here is deep and very clear, with good snorkelling over rocky ground where sea urchins, small fish, and the occasional octopus go about their business. There is no beach as such — you swim from flat rocks or from the steps of the Perdika boat hire places — but the setting, with the islet of Moni visible just offshore, is hard to beat.
The area around Marathonas, just north of Aegina town, has a long shallow bay that is excellent for families with small children because the water stays warm and knee-deep for a good distance out. Less well known is the cluster of coves just south of here, reachable by following a dirt track off the main road. These have no facilities whatsoever, which keeps the numbers down through most of the season.
If you hire a small boat from Aegina town or Perdika — which is straightforward and not particularly expensive — you can work your way along the coastline and stop wherever looks good, dropping anchor in a cove you have entirely to yourselves. This is perhaps the most satisfying way to swim on the island, and the boat hire operators will mark the best spots on a simple map when you set out.