Came to post this. These things are frigging amazing and like $20. Absolutely everyone should have one. Really you can floss and brush and it still gets stuff. I wish they existed when I was younger! Buy one! Thank me later!
On the 'dentists don't really know what they're doing' front, my kid's orthodontist ($$$$) describes himself as a "fencing contractor". Conversely, I got the water flosser tip from an Indonesian dentist who I hired opportunistically on holiday for a clean; he turned up in flip-flops but decanted to bare feet during operating and gave the most sage advice I've ever had in 40+ years of dentists: BUY A WATER FLOSSER. Terimakasih, Doktor!