I've had the same strap (old style 'solid' type) since I got my 705, and use it now with my 800. With both of them it can sometimes take a bit to get a good 'signal' from my heart and give an accurate reading, sometimes being too low until I've sweated a bit for a connection. It can also be ridiculously high (120%+) which is apparently static from dry synthetic jerseys before the contact with the chest is good enough to dominate. It's been exactly the same with both units.
BTW, I can attest to the waterproofing, having accidentally washed the strap at 30 deg. accidentally a couple of years ago, which had no effect!