Sunburn
Cretan sun has quite much burning power, which you should never underestimate. It can be punishing as early as April. You should use a high-factor sunscreen or sunblock, especially on boat trips.
Heatstroke
Do not attempt strenuous hiking or mountain riding in July or August, drink plenty of water and wear a hat. Symptons can be aggravated by alcohol - drink an extra half litre of water for each unit of alcohol you consume.
Mosquitoes
They breed in ponds, streams and cicterns. Out of doors, they can be kept away by deterrents containing diethyltoluamide ('deet') or the organic citronella oil. Indoors you can use an electrical deterrent pad.
Sea Urchins
Spiky black sea urchins infest most rocky beaches and can inflict a painful wound if stepped on in bare feet. You should watch where you walk in shallow water.
Jellyfish
Jellyfish or Tsouchtres sometimes can inflict a painful sting. Rinsing with vinegar or a mild ammonia solution is an effective remedy, and an antihistamine tablet and cream will also reduce inflammation.
Snakes
In Crete there are many snakes, however most of them are harmless and disappear when humans approach.
Kamakis
Greek kamakis ('harpooners') have become a myth from the late 80's, since they are locals who are willing to offer a holiday romance to most of the girls coming from the whole world!
Hornets
These large, red and black cousins of the wasp inflict a very painful sting and should not be handled or provoked.
Photographing Military Installations
Greek authorities are extremely sensitive on issues of national security, as a group of British plane-spotters discovered in 2002, when they were arrested and convicted of espionage after taking photographs of Greek Air Force Warplanes. Do not take photographs of military airfields (which often adjoin civil airports), army bases or radar installations (they are dotted around Crete's coasts and mountains), or naval vessels (even in civil harbours), or you are likely to be charged with spying. Prohibited sites are signposted with the symbol of a camera barred by a red stripe.