How to lose weight?

I cannot work out outside in the summer due to my high blood pressure and asthma unless it's in a swimming pool, but I am open to try other things. Also, all I drink is water. Edit: I don't have a car for the gym, and the gym near me said the membership was $10 a month. They took $80 out of my account for that month though. I went with my sister and her friend. They took out $80 on their's too. We got our money refunded, but I don't trust them to try again.