As far as medical science is concerned, there is no relationship of temperature of food to heart disease; nor are fruits like Banana, Pineapple, Water melon contraindicated in patients who have a hole in the heart or valve defects. The only thing I can imagine that could go wrong with water melon is that if you eat water melon in plenty, the water load may be excessive and in case your heart is in state of failure, then it can create problems but otherwise water melon has got no harmful effects on the heart. However, I would advise you to restrict your salt intake and to some extent water intake also.
Your query regarding common cold & cough. Well I do not have all the medical details but in patients with hole in the heart, there is increased flow of blood through the lungs. They are therefore prone to respiratory problems and recurrent common cold and cough. In that respect, I would advise you to get in touch with your cardiologist and should definitely consider surgical option for closure of hole in the heart and repair of the valve if that is still possible. If there is any local problem in the nose in terms of polyp or deviated nasal septum which may be causing problem then that may be remedied by an ENT surgeon and therefore I would also advise you to consult an ENT surgeon.
Your last query of precautions for not getting Asthma attacks - well one should avoid dust and pollution and specially during change of weather, one should be little cautious of not getting exposure to sudden cold or sudden change of temperature. Also one should try to find out any specific thing to which one is
allergic, for example pollen grains and avoid that. I think a chest physician or a respiratory specialist may be able to help in this matter to identify your problem.