Though crime rates in India are way below those of western countries, precaution should be taken in crowded places, and valuables should not be left unattended on beaches.
Bottled water, available even in the remotest places, should be resorted to, as tap water is not potable in India.
Mosquitoes can be a problem in many parts of India so carrying insect repellent is always handy.
Power supply can be erratic so it is a good idea to carry a small torchlight.
It is useful to check permission for photography in most places of tourist interest, illegal photography could be punishable.
Bargaining (except for household items, food, cigarettes and medicine) is almost an accepted practiced. Never worry about the original price quoted, you could end up paying a third of it.
In cities, women will inevitably experience at least a session of eve teasing. It is advisable for single women travelers to dress decently and avoid staying out late particularly in quiet and dimly lit areas.
Food is eaten with the hands. The right hand is used to eat food, and also to give and receive things.
Religion is taken very seriously in Kerala and respect must be accorded where due. Some temples restrict the entry of non-Hindus.
One should dress modestly as Keralans are generally conservative, and respectable dressing creates a good impression.