How bad is this? (back glass cracked, sensors seem intact)

As long as it’s still fully functional, I wouldn’t be too worried about that scratch. Apple has never claimed that their devices are waterproof, and suggests the use of the “water lock” feature when doing things that will get your watch wet. As it comes to repair, it’s going to be extremely difficult to replace that. If you take it to Apple, they will probably try to sell you a new device. As long as you’re not too worried about the scratch causing water damage, it shouldn’t be too big of a deal.