How to clean and polish a composite kayak?

What products are recommended to achieve a high luster finish on my Nelo 520 WWR (4)
Carbon/Kevlar Epoxy Ski?  It is only one year old and used exclusively in a fresh water lake but the finish has dulled slightly over this season (hard water spots) and I want it restored to the same brand new and pristine factory gloss finish it was born with.

I, too, own the same model and layup of ski, and so far it has only been paddled in fresh water. I hose off the ski and wipe it dry after every use (live on a dirt road and do not want to leave dust on it to get ground in on the next drive).  But even so, there is some slight dulling of the finish. The ski has been in use since spring of 2020.

All of my sea kayaks got the dulling effect over time, whether varnished wood or gelcoated fiberglass layup, or rotomold plastic. It was worse at the house where we had well water, which definitely was hard water.

I have not yet tried this, but it occurs to me that since the problem does seem to involve hard water deposits, maybe a mix of vinegar and water, applied with a soft cloth and gentle elbow grease would get rid of the mineral film. Just don’t get it on any metal hardware.

I ordered my nelo 550 with matte finish in part for that reason.  Hard water spots and micro scratches dont show. 

I have a WWR 520 that I paddle at least 6 days a week on a salt water river with a lot of tannins and other dissolved muck that stains the hull. Which is gloss white. Never rinse it off, just pull it out of the water and hang it under the deck.
Try this…the FSR works really well on stained gelcoat…looks like new with essentially no elbow grease. 

kayakdave.com/2012/06/02/how-to-wash-cle...h-a-composite-kayak/

 

You answered you own question in the title!
People think kayaks and skis are self cleaning, well as it turns out they are not! Just like your car or even you own body, just see how they look with just dipping yourself in water every day or hosing the car off, I would suggest you keep well clear of any other people, but it put it to you they might keep well clear of you!
SOAP! 
With one addition to a composite finish you can add, polish. A good quality car polish will do the trick if it's painted or and much courser cut and polish for gelcoat. Gelcoat is much more durable and can take much rougher polishes.

Pretty basic stuff but to remove stains it sometimes takes more than polish and elbow grease, sometimes chemicals are necessary. That’s where FSR comes into play, I have the ubiquitous Nelo 520 with gloss white gelcoat on the waterline. I paddle in a salt water river with a lot of tannins and the boat picks up stains, the FSR obliterates them and doesn’t require sanding layers of the gelcoat off to do so. Here’s a nice tutorial.

kayakdave.com/2012/06/02/how-to-wash-cle...h-a-composite-kayak/

I am considering a Nelo 550 ML or 540 ML. I demoed the 550 on flat water and plan to use the craft in mostly sub 15 knot winds so smaller conditions. I see you have owned other skis. The Nelo seemed to fit me well, but I wanted to ask you how you decided on the Nelo after owning those other boats. Also, if the 550 felt good to me (the 560 was a bit too tippy), would that be a safer choice than the 540 so I don't have to try to sell a boat after a year of paddling 3-4 times a week on the flats or small conditions? How do you specify matte finish when special ordering? Does it look nice and does it pick up more dirt or affect the drag as far as you can tell? Sounds like a good idea it scratches and other superficial damage is obscured by the matte.

It generally will not affect the performance.
However you can't polish a matt finish, and if you do it will just become shiny as that is how the process works to get a gloss finish.