there's plenty of GoPro clones on Amazon...and clones of clones. Just read the reviews, make sure you get the same product in the mail as what is described. I'm pretty happy with a camera I paid 60 dollars for 2 years ago and which can take GoPro accessories. (not the case, the camera usually comes with its own waterproof case, but everything that fits on the case should be compatible with the GoPro).
I think the main differences between the clones and the
GoPro are the inability to record at high framerate , lower resolution (but plenty for your needs) and the sensor is lower quality, but, again , plenty good for action footage.
For surboard mounts, make sure you wait 24 hours after application before you hit the water. A tiny leash with its own mount is recommended, and an orange floating sponge to glue to the back of the case. All these accessories are really cheap online and usually big bundles of different mounts and accessories can be had for less than 20 dollars on Amazon.