At present, Apple's WatchOS won't connect to Wi-Fi or iCloud directly. Even if you brought up each image and took a low resolution screen capture - there isn't a viable way to send them off the device in the absence of the phone to which it was paired.
I would do two things just in case there's a maintenance procedure for this:
- Contact AppleCare and ask about watch backups when the phone is gone.
- See if waiting for the phone to dry out over a week or more and/or risk opening it to expedite the drying process.
Most devices that get submerged will function for a short time (with greatly reduced lifespan) once they dry completely - and that process could take a week or more before you introduce it to the best chance of working (known Wi-Fi network, connect to power to see if it boots and then connect to iTunes in the presence of the watch and a pre-joined network to try and get a backup of the photos).
Sorry I can't offer a higher chance of success. Going forward, you can make sure photos back up to iCloud and get the phone to network access periodically to reduce the window in time where photos only exist on the phone/watch combination.