You shouldn't use a fresh water test kit for testing salt water. The chemicals used for each are different.
Here's the test kit I use - tests both fresh and salt water. There are different bottles for fresh water and salt water and different color charts for each as well (for pH for sure). It shouldn't be over $20 at a big chain pet store.
Tetratest Laborette Test Kit (Pond Supplies > Water Quality Testing > Test Kits )