My guess is it would act just like an extension cable, just one bolted to the wall and waterproofed. All other previous caveats would still apply (it's what I would do, but I am not an electrician). I had a single one (a proper socket on the kitchen ring) and bought a double that I got the electrician to fit when he was doing the EICR. Suuuuper handy.
I have done exactly this to run an extension out for use when we have the pool up during the summer, for extra protection I whacked the cable in a bit of ducting I had lying round. I just then unplug it when the pool is not in use.
My guess is it would act just like an extension cable, just one bolted to the wall and waterproofed. All other previous caveats would still apply (it's what I would do, but I am not an electrician). I had a single one (a proper socket on the kitchen ring) and bought a double that I got the electrician to fit when he was doing the EICR. Suuuuper handy.