Assuming you have white topsides with blue non-skid and the blue gel-coat is worn away showing the white underneath.
Spray guns aren't that hard to set up and once set up can quickly deliver product.
I suggested spot treating with gel-coat because it preserves more options than paint if you want to expand and do larger areas.
There are services that will match your gel-coat for you. I'm not very good at color matching, so I would probably spray an entire section. The problem with spraying over the non-skid surface is that you will partially fill the non-skid surface.
Gel-coat will not fill small cracks, they need to be addressed ahead of time.
I think you could spray gel-coat and if you think it filled in the non-skid to much, spray a second coat and sprinkle some grit into the wet gel-coat.
Another option is to look at kiwi-grip and stick on non skid sheets.
Guy