Believe it or not, I didn't use a primer but used
Dulux Quick Dry Satinwood instead, on the cabin of
Just 17 and I was well pleased with the results. It behaves very much like an emulsion, with little to no smell.
Just make sure the surface is cleaned well, has no flaking paint on it, and the marks I see in your photos are just the bare GRP, not mildew.
You'll see the Dulux web site only mentions usage on wood and metal, but I thought I'd try it as it claims self-undercoating and the can says it's touch dry in 30 minutes, which proved to be true. It recommends two coats. You can apply the second in six hours. That means you can spend the time waiting to apply the second coat doing any other work needed to be done in the cabin and at this time of year the job can be done in a day if the surface is already prepared.
It's also available in 750ml cans so you don't have to buy too much.