Sea water is ~3.5% salt. Most salted paper recipes I've seen call for between 1.5% - 2% salt, so diluting sea water 1:1 with distilled water would bring it into the optimum range. IIRC, higher salinity produces a great loss in sensitivity and maximum density.
My "Getting Started in Wetplate Collodion Photography" article:
Ive been getting into salt printing lately and have just two questions I cant seem to get answered. First, anything Ive read about using ocean water says to dilute it at least 1:1 with distilled water. Is this really necessary? What would happen if the salt solution was as high as %30 ? Also Ive been using gelatin with my salt solution but have seen recipes that are purely salt. Which papers are best for such solutions where no gelatin is used?