Since cloth is a porous surface, it will absorb water unless sealed with a non-pourus surface, the main ones being metal and plastic (though plastic can be puorus at extreme pressure). Since I imagine that you do not want to coat your seats in metal or plastic, I think the best solution is to get some good nylon/plastic seat covers, or put your top on...
With the manufacture of hi-tech cloths using nano-technology on the horizon, it will not be long before a cloth is made that will withstand water a great deal. Several companies are already using this technology to make their jeans much less prone to damage from spills. I read an article about it somewhere, will look tomorrow at work and post the link if anyone is intrested.