I'll go this weekend to a village called Louro, I'll tell you how it is right now.

I've been following the Noticias on TVE via Dish network, and they've been updating the Prestige situation daily. It looks like the cleanup continues, but headway is being made.

The one thing that is still a problem is the damage in remote areas of the coast. The report later from CanalVasco (ETB) mentioned that in Biarritz, they're still seeing periodic washups of petroleum. (Biarritz is in the French Pays Basque just in case you aren't a walking atlas )

The beach in Louro looked clean, there are small drops of oil that the sea leaves in lines where the waves finish at the different tides, but you can only see them when you have them on your feet, the general view is the one of a clean beach. In the rocks was different, there's a wide band of rocks covered with petrol, the rocks closest to the sea are clean, but those where the sea arrives only in high tide are still black. In a first view it could look that black is their real color, but if you get closer you see it's petrol. The ria of Muros lookes clean also.
Anyway the beach was beautiful, it still worths to see the landscape. The water had an incredible blue color and the people was great and very thankful and lovely with all the help and shows of solidarity.
I don't know how it'll be in May as there must be some oil yet somewhere in the sea, and not all the places are the same. But for painting and seeing the landscape I would recommend to visit the place. I have heard that Rias Baixas are clean as the Islas Cies stopped all the fuel.