The Northern Aegean Islands are the islands of the northern part of the Aegean Sea. They can be divided into two groups: the islands of the Thracian Sea (ie the northernmost part of the Aegean Sea), and the Northeast Aegean islands.

Islands in the Sea of Thrace, map

Above the 40th parallel, the islands of the Thracian Sea are the most northern, and have a transitional Mediterranean climate, characterized by fairly cold winters (with possible outbreaks of cold air from the Balkan Peninsula, associated with snowfall and frost), and hot and sunny summers, but shorter than in the southern islands; they are Samothrace, Thasos and Imroz (the latter belonging to Turkey).
Here for example, the average temperatures of Samothrace: in January and February highs hover around 9/10 °C (48/50 °F); summer is hot, since highs in July and August are around 31 °C (88 °F), however, already in September the temperature drops by 5 degrees Celsius.
Average temperatures - Samothrace
Min (°C)335913182021161385
Max (°C)91013172328313126211511
Min (°F)373741485564687061554641
Max (°F)485055637382888879705952

The rains are not very abundant, since they hover around 600 millimetres (23.5 in) per year, and follow an almost Mediterranean pattern, since they are more frequent in winter, however, in late spring and summer some thunderstorms can occur, coming from the continent, especially in Thasos, which is closer to the mainland.
This is the average rainfall in Thasos.
Average precipitation - Thasos
Prec. (mm)706050455040302535557090620
Prec. (in)2.82.421.821.

The amount of sunshine is excellent in summer, when clear skies prevail, and it is also good in May and September, while in the rest of the year it's not so high, since there are several periods in which the sky is cloudy. As mentioned, in the northernmost islands, some thunderstorms can break out also in the middle of summer.
Here, for example, the average daily sunshine hours in Thasos.
Sunshine - Thasos
Sun (hours)456891112119754

The temperature of the sea in the Thracian Sea in summer is even slightly higher than in the Aegean Sea, because the latter is affected by the Meltemi, a cool wind blowing from the north and able to blend the waters, thus reducing their heating.
Here is the temperature of the sea at Thasos.
Sea temperature - Thasos
Sea (°C)131313141822242523201715
Sea (°F)555555576472757773686359

Northeast Aegean islands, map

The Northeast Aegean Islands, lying further south, become progressively warmer in winter, and less subject to cold spells, while summer becomes progressively longer. They are, from north to south: Lemnos (still pretty cold in winter, given the northern location), Agios Efstratios (or Saint Eustratius), Lesbos, Psara, Chios, Oinousses (or Aignousa, small island to the east of Chios), Samos, Ikaria (the westernmost, a short distance from the Cyclades), and the Fournoi Islands (or Fournoi Korseon), namely Fournoi, Thymaina and Agios Minas.
Here for example the average temperatures of Lesbos.
Average temperatures - Lesbos
Min (°C)77811151922211815129
Max (°C)121315192428303027221714
Min (°F)454546525966727064595448
Max (°F)545559667582868681726357

As mentioned, the temperature of the sea in this central part of the Aegean is never very high, in fact it does not exceed 23 °C (73 °F) in July and August.
Here are the temperature of the sea at Chios.
Sea temperature - Chios
Sea (°C)151515161822232322201816
Sea (°F)595959616472737372686461

The amount of sunshine in the Northeast Aegean Islands is excellent in summer, when clear skies prevail, it's very good in late spring, while in autumn and winter, sunny periods alternate with others characterized by clouds and rain.
Here are the average daily sunshine hours in Lesbos.
Sunshine - Lesbos
Sun (hours)55781013141311854

The best time to travel to the Northern Aegean islands, for swimming and sun bathing, is the summer, from June to August in the islands of the Thracian Sea, and from June to mid-September in the Northeast Aegean Islands.

Back to the Greek Islands.