Re: wood swim platform
If its teak, which they usually are, you have 2 options, clean/brighten/oil, or clean/brighten/seal/coat with a spar varnish. Both have their goods and bads. Well maintained oiled teak is beautiful, but it has to be maintained on a weekly/monthly basis, and its dificult to maintain the underside if left in a slip. Oiled teak is also safer on a platform as it has better non skid properties.
Varnished teak platforms are beautiful too, and require much less maintenance. There is a higher skill level required to get a glass finish, and much sanding is needed between the 8-10 coats of varnish. And a good varnish needs a while to set up, between coats and sanding. A varnished platform has little non skid properties, and can cause injury or worse.
Im still deciding which way to go myself.
Epifanes over clear shellac is a good choice for varnish, any 3 stage teak kit for oiling.