Taking a warm sea salt bath can actually lower your blood pressure instead of raising it so if you're susceptible to low blood pressure then it would not be recommended to sit in a sea salt bath longer than 30 minutes.
If you do have low blood pressure then it would be best to speak with your doctor about the sea salt bath to make sure it's not gonna do your body and healthy any harm.
But as long as you don't soak in the warm sea salt bath longer than 30 minutes it shouldn't do you any harm although the blood pressure does get lowered usually but it's only temporary and your blood pressure should return to normal within a 10 to 30 minutes after getting out of the sea salt bath.