RIT (the dye people) also makes a product for washing your clothes. It makes them have a high SPF, so if you can find it in your area, it may be more cost effective than the special SPF type clothing. Also remember to wear a wide-brimmed hat. I feel for you, I have photosensitivity from my diabetes med (glucophage, it's not common so go figure, but overall it still is the best choice for me). I get itchy red blotches as opposed to an actual burn, my dr says it's like sun poisoning. I have found that being on the water in a boat is not good regardless of what I do (block, clothing, etc.). I think it's the reflection. If you have any particularly sensitive areas, Dermablend makes makeup (both for the face and for the body) that affords the same protection as zinc oxide but doesn't look weird. Obviously not a good idea for your whole body, but a lifesaver for small sensitive parts when you know you will be outside a lot (the backs of my hands are a particular problem for me). We also plan our days to minimize the amount we are outside during peak sun hours (late morning/early afternoon).
The comment about making sure that you put on enough block (and reapply) is also right on. My dr says that most people don't get the full coverage bc they don't put on enough block. I have ordered from drugstore.com bc the selection of suitably high blocks here in the midwest is poor. I like the spray-on type for reapplication during the day, it doesn't tend to get gooky.
Finally, I have found that slathering on fresh aloe at night really helps me, it is the only thing that does anything with the redness/itch. To pass on a feel good story, we were in a drugstore in Hawaii trying to buy yet another cream (this was after the a boat trip, big mistake, the 60 SPF that we had such high hopes for didn't do the trick). The pharmacist sent us to a nursery where we showed them my arms. They gave us an aloe plant and just refused to accept any payment. It was like a miracle, the next day it was so much better.
We vampires have to stick together!

I know I said this before but ... what a great board! I have never met anyone before with photosensitivity.
Oh, remember that SPF simply extends the time that you can be out in the sun w/o burning. With photosensitivity from meds, you may not be able to tolerate any sun, so X times nothing is still nothing. I learned this the hard way, I still liberally use sunblock but I don't rely on it exclusively.
Cupcake