"Kēhau is hot."
Translation:Wela ʻo Kēhau.
"Ke ikiiki o Hilo!" (note the lack of 'okina on o) is saying "The humidity of Hilo!" The o marks possession, and you're saying an incomplete phrase as an exclamation of how humid it is. "Wela 'o Hilo" (note the 'okina now!) is saying "Hilo is hot". The 'o marks Hilo as a proper noun and you're saying a complete sentence.
Ka wela o Kēhau would be "the heat of Kēhau." Translating Ka ikiiki o Kona as "Kona is humid" would be a non-literal translation. It's like if someone on a summer day in Atlanta remarks "Wow, this Atlanta heat" and it was taken to mean "Wow, Atlanta is hot." Similar information is conveyed, but the wording is different.