That's odd. It's mostly teens these days, but also larger mom's (at least in Edmonton) that wear thongs to the public family swim. There's no snickers, it's 100% accepted. That goes for my thong too, there's no laughs.
There's often 1-2 (not many) speedo wearers, so it's not an uncommon sight from the front, and cheeks are common place now. Don't need a thong board!

Not the Boise River but I have done a few inner tube float trips on the Kankakee River in Illinois. The first one was in a pair of jean cutoffs. Like in my teenage days I cut them so short the front pockets hung below the hem. That was fun but there's that whole sitting around in wet clothes thing happening. The second trip I decided to thong it. We got to the park we launch from, blew up the tubes and when it got time ti leave I dropped my shorts. Kind of surprised the wife but she was happy I decided to wear anything. (I'm a nudist). As soon as the shorts dropped some loud mouth screamed, "there's f*cking kids here!" I replied that he should watch his mouth around kids and that we wouldn't be around since were leaving to go float anyway. Once we got on the water it was great and much better than last year with soggy denim. I alternated between being face up with my butt in the hole and on my front stretched across the tube in order to get some sun on my cheeks. While I was face down I heard some splashing in the water and when I lifted my head there were 3 canoes coming up on us. I went to turn over when a voice came from one, "don't turn over on our account, we've already seen your ass!" They caught up to us, we exchanged some pleasantries, nice current today, how long since you put in the water, and where are you taking out at... that sort of thing. When they said they left from the same park we did and did that trip in 45 minutes when it took us close to two hours, I told them to throw away those paddles.. they're going too fast. They had a laugh, one of the women took my picture and they paddled away. We stopped on a sand bar for lunch and not having seen anyone in a while we went skinny dipping and ate our lunch while still nude, With lunch and a few beers in our belly we got dressed and continued down the river. We passed a few restaurants and bars along the way but nobody paid us any mind as we floated past...even with me and my exposed butt. Finally we got to the end of our float where we left the other car. It was at a VFW hall and it was a lot more crowded than it was in the morning when we dropped my wife's car off. We got to the side of the river when my wife asked if I wanted to cover up. It had been such a nice day I told her I was good and would stay dressed as I was. We made a few trips between the river and her car and all I got was a wolf whistle and a few laughs. Once we got the car loaded, I opened a door and took the thong off (behind the open door) and wrapped myself in a sarong for the drive to get my car and head home from there. All in all it was a great day on the water. On our last trip I wore a g-string but went nude for most of the float but did not go butt-up.