I came here for the Ironman Triathlon so it is a busy week and almost all accomodation in Kailua-Kona are fully booked.
Jessea's place was great (outside of the Kona fever during race week).
However you need a car or moped as it is probably 20+ min ride from downtown (i.e. you need to get to registration, bring your bike, get to the starting line...)
Kahaluu beach is not far (maybe 10min drive) and great for snorkeling and longboard surfing.
Magic Sands beach nearby as well.
And can drive back to town via scenic Ali'i drive.