Cape Verde can get very hot during the summer. But it is very, very different than summers in the southern United States like Florida. Southern Florida is extremely humid during the day, and worse, during the nights too which makes it difficult to sleep. Air conditioning units are a requirement to be comfortable in Florida in the summer. I had a summer apartment in Florida. If you are not used to humid climates, Florida can be suffocating. It is made worse because of all the development. So the cars, the asphalt, all of the buildings ... it makes it very difficult to feel comfortable. This is why everyone in Florida has a backyard pool.
Summer in Cape Verde is nothing like Florida. It can get humid in the summer here in Cape Verde, but is not as humid as Florida. The islands are small and surrounded by water. So there is a lot of cool sea breeze. And there are not as many cars and asphalted roads to radiate heat. Plus the sky tends to be cloudless. Less clouds mean there is less heat trapped in the atmosphere, so the nights can get cool very quickly. The key is to stay out of the sun. Direct sun can be extremely hot.
In addition, some of the islands are drier than others. For example, Sal and Boavista, where you are planning to stay, are deserts. No water. No humidity. But the sun can be extremely hot and there are very few places to shade because there are no trees or vegetation. At midday, you should stay indoors if you are not used to very hot sun. Use lots of sun-block lotion too.
Your other question was about E. Coli. Most of the water produced in Cape Verde is from desalinated plants where impurities are filtered out and the water is treated, typically with chlorine. The water exceeds the WHO international standards for human consumption. Still, I do not drink the water unless it is boiled first. So I will use it in cooking, in coffee, etc. But that is not because it is Cape Verde. I do not drink water anywhere in the world unless it is boiled first. That's just me. But I have never heard of any E. Coli outbreaks in Cape Verde due to the water.
For normal drinking, I use bottled water. If you pay more than €1.60-€1.80 for a 5 liter bottle here in Cape Verde, then you are being ripped off. That's what you pay in Praia. Sal is probably a bit higher because of all the tourists that go there.
There is a local company, Trinidade, which is owned by Tecnicil Industries, that produces bottled water here. There are also lots of imported brands.
