The heart monitor technology used via diodes on these smart watches isn't very accurate. I can't seem to find the article I was reading but I remember a roughly +/- 10bpm error rate. That's also roughly what I've experienced myself on the LG and Moto watches I've tried.
That said, if you buy a Garmin or similar heart tracker watch, they package it with a Heart Rate monitor transmitter you strap around your chest. Garmin would love to be able to cut costs and keep everything in one piece of hardware, but the prosumer crowd who buy this hardware for Triathlons etc would be furious if the product was inaccurate.