I have been selling window coverings for 33 years now. For a marine application - needing to lower heat - and still see out, the solar shades are your best option. All the components are aluminum except the chain which is nylon and plastic. The gear mechanism is plastic with a STEEL spring which is the only rustable part, however; the spring is totally enclosed and hidden. The fabric is vinyl coated fiberglass and polyester and was designed in Europe, originally for outdoor use. I would suggest a 3% to 5% openness style, which includes the 2000, 4000, 4400 and some of the 5000 series.Any openness more than a 5% won't be adequate for the heat in a marine environment. While some of the above mentioned styles may have the same openness, weaves type varies by style and makes it a personal preference as to which weave you like best and has your desired color need.
Having done some marine installations, sometimes the primary problem is determining where a solid place is at the window for the mounting brackets, which should be determined when measuring.