Call the fire brigade, or more likely (unless anyone is in immediate danger) a good recovery truck.
Then, first of all, ANCHOR it securely, either to several of those handy trees, or to the recovery vehicle (which hopefully will have it's handbrake on, and some hefty wheel-chocks!). If you tried to lift it before it was anchored firmly, there's a chance that you might alter it's balance just enough to send it into the water, which would be very embarrassing!
You could then try lifting it (but only if you'd decided that it would help). From the photo, it looks as though you'd struggle to get trolley jacks under it in a position that was far enough forward to lift the front end (although you MAY be able to jack a couple of inches, insert chocks & blocks, then reposition the jacks a few inches forwards, jack again, & repeat a few times until you raised the front end). A decent recovery truck, or fire brigade rescue vehicle would carry a selection of inflatable airbags, that should fit easily enough under the chassis, directly below that front o/s window. You could raise it with the airbags, chock & block it, then replace the airbags with trolley jacks or similar to allow it to roll as you winched it back onto the quayside.
If it was up to me, I wouldn't bother with ANY of that jacking or lifting. It would give very little, if any advantage, unless, for example, the engine's sump, or transmission housing was hanging so low that it prevented you from winching back in that position (because it would jam against the quayside), OR unless you judged that FURTHER serious damage may be done to the vehicle otherwise.
I would then get someone to keep the owners occupied, or turn the radio up to full volume, so that they wouldn't be alarmed by the loud scraping noises as I winched it quite easily back onto the quayside!
Of course, you could try getting a friendly truck driver to tow you straight back, but generally winching is much safer and much more controlled.
Alternatively, I would just phone the motorhome hire company, and tell them that I've changed my mind!!

