It sounds as though you have given it your best efforts. Don't give up, and make sure that the strength is similar to the UK sea areas.
I have to say that I don't know the salt ratio of typical sea water, but Wiki says
http://en.wikipedia.org/wiki/Sea_water
At home in Essex, it never seemed particularly strong, but here in the Bay of Mexico this summer it really did sting my eyes and the little'uns seemed to get red eyes, so it was pretty strong.
The issue of Iodized salt was discussed on this forum a while back, I would guess that it would be better for your problem, but there may be other issues. Take professional advise.