1. You're up against a sine wave and the reality that, in England you get 10% of the power in the winter in the summer. So one question is; what do you do with the other 90% in the summer if you meet your winter needs?
2. Solar energy input and boat electricity usage is not an exact science where you can tweak the numbers by a few percent and expect to measure the gains or losses.
3. Space is usually a limiting factor, except on fat flat boats, so simply go for as much as you have roof space for in a reasonably cheap panel format. Chasing diminishing returns is a mug's game because of 2. above.
4. For a limited use boat such as you describe 24 amp hours which is a fridge in the summer and lights in the winter, 250-500W seems a good place to aim at, it also puts you in the range of affordable controllers.
The low light performance is, in my experience, a myth. Amorphous panels were originally designed for the American military with durability in mind. They are very robust, I have a 64W with a big scratch in the plastic that performs exactly the same as one without.