I've always like Summerland if there is anything available.
Very short drive on the highway to Penticton and it has an aquatic centre.

did you try phoning some of the places on lakeshore?

a lot of them dont even have next summer available online yet - but they did do the bookings for ironman over the phone/email

There's an accommodation thread on the facebook group that has rentals being advertised as recently as yesterday: https://www.facebook.com/...nmancanadapenticton/ (scroll down)

Yes, we have stayed at Summerland and also rented a nice cottage on Naramata one year, I don't know about now but 15 years ago a lot of those motels downtown were nasty and expensive

I always found most accommodation in Penticton overpriced and poor quality.

I've made reservations months in advance in Penticton, and the last two times I called the week prior to the race. I was able to get three days/nights at reasonable rates calling at the week before. Make a list of all the motels/hotels in Penticton and call them directly now or later next summer.