Welcome to the Recel small family.

For the mylared playfield, there is nothing to do specifically. Just keep it clean, using alcohol or any gentle cleaner of your choice.
A Recel machine plays naturally very fast. There is absolutely no need to wax the mylared playfield. As you will see, with a clean and fresh ball, these machines are among the fastest electro-mechanical pinballs you will ever see.

As Ken indicated to you, most parts are directly sourced from Gottlieb. Your plastics seem to be in excellent shape and you have all the little white hat to hold them. I do not know if these "hats" are easy to find, so it is better to not lose any.

For details about Recel machines, I have published a Blog about the Recel Lady Luck. That may help you in rewiring your machine:
https://pinside.com/pinball/forum/topic/ycv-recel-lady-luck-restoration-my-first-em#post-3266805

Yves

I am sure you checked the IPDB database:

https://www.ipdb.org/search.pl?any=underwater&sortby=name&search=Search+Database&searchtype=quick#2702

It has some nice picture of Underwater. After seeing the backglass, it came back to my memory that I had played that machine in France, in some cafes. I have always had a strong interest for submarines and that backglass definitely attracted me, when I was a kid.

For the rubber, just measure and buy the corresponding rubber rings. There is a drawing (unfortunately incomplete) of the playfield in the data base.

Yes, please start a BLOG and tell us about your progress. There is a gentleman describing his "Whack a Mole" pinball and We would love to read about your endeavor with Arduino and other Micro-controllers. For sure, it will be a lot easier than trying to rebuild a mechanical machine. If you decide to do so, you should move your BLOG to the restoration files.

All the switches and most parts can be sourced from Gottlieb machines (the Pinball Resource or other local donors).

Good luck.
Yves