In complete disclosure, my Galaxy watch has a glass back to it, but I wear my watch 24/7. I have learned, the hard way, that I must wash the back of my watch (it's waterproof) at least twice each week, and I use an anti-bacterial soap when I do this. If if don't, and don't rinse well, then I begin to get contact dermatitis. So, please be aware of that. Also, all smart watches us a powerful 'LASER-like' diode to put light through your skin so that the sensor can read what is happening under the skin. Weirdly, the lastest Galaxy watch will also read the skin, itself, and do a photo-assay of the free radicals in your skin (this is to see if you're getting enough anti-oxidants in your diet to maintain health....!!!). But I feel it incumbent upon me to let you know this if you have sensitive skin.
What I have done, and it doesn't seem to affect the function of the watch's sensor, is to apply a light 'skiff' of 'barriere cream' on my skin or on the back of the watch, but not normally...only when I begin to get a skin redness under the watch. My dermatitis fixes itself inside of 24 hours doing this.
RF should not affect heart rhythm. The frequency is too low to impart any useful voltage to nearby tissues. Also, RF catheter ablation is still widely used (on me twice in the past three years) to ablate tissue causing atrial fibrillation. If it were true that RF can cause arrhythmia, it wouldn't be useful. It DOES damage tissue, but only when the RF wand is held against tissue and energy applied....which is what ablation is.
Remember, Bluetooth and other forms of emissions are around us all the time, and this includes radio wave broadcasts. Their frequency and energy content are far, far, too low to affect human tissue.