Try to avoid dark paths and be careful not to leave your bag unattended. Swimming alone on secluded beaches is not advisable. If you are on a boat, you should avoid taking valuables with you, or if you have no choice, be familiar with finding a nice hiding place.
There is some sketchy activity along a shady side road behind Beau Vallon beach (left of the Boathouse restaurant) on Mahé, but locals seem mostly content to admire their swanky cars and mostly ignore passersby.
There is a newly established tourist police force stationed on every beach on Mahé, easily recognized by their blue or white golf shirts with a tourist police badge sewn on them. They are very friendly and more than willing to turn a blind eye, even though you may not see them. They are honest and willingly give advice. Potential thieves are conspicuous ( most likely because there are locals lurking and noticeable) and they tend to hide out in the immediate vicinity of the beach or in the tight streets close to the more isolated beaches. Prison sentences are harsh and strictly enforced, as the island makes a lot of money from tourism.
Chikungunya virus is a mosquito-borne disease that causes flu-like symptoms. It is of increasing concern and although it is rare to die from it, the joint pain it causes can last for months. Insect repellents can help deter mosquitoes, but not much else can be done as a precaution. The disease is native to East Africa and is occasionally introduced and quickly eradicated.
Tap water is safe to drink in most areas of Seychelles, but water quality varies in undeveloped areas. It is recommended to drink bottled water only and avoid bodies of water such as lakes, rivers, ponds, etc.