My tips would be to take an antihistamine ( not knowing how long these stay in the bood stream, I would start a couple of weeks before exposure). If possible, for sunbathing, use a protection that is not waterproof. I had a huge problem with this type of allergy years ago, but when I switched to a non waterproof product it helped immensly. At the time I used a spray that was very "watery", unfortunately this has now been discontinued as it was a low factor. The newer products on the market do seem to be much lighter in consistency and I now go for the wateriest I can find. Obviously to swim, a waterproof is essential, but I towel it off afterwards. I feel that the waterproof ones tend to clog the pores. Since doing this I haven't had a problem.
Years ago, when I did suffer, I found that in some countries it was possible to buy a talc which contained an antihistamine and I found that this did help to soothe the rash. When I asked a British pharmacist about this , I was told that it wasn't available in England as it contained lanolin and this could cause allergy. However if you know you can tolerate lanolin, it might be worth trying to get something similar. Good luck to anyone who suffers from this, I know how awful it can be.