For the first question, you can use two methods.. To drink salt water you need to get rid of the salt first. and you can achieve it by using a water vapor condenser. Now when you are in sea your not privy to a readymade condenser. So you have to make one..

The most important element  in making such a thing is a see through plastic sheet. or a metal sheet or a metal pipe. if you have none then its better to stop doing anything, pray to god and look for land. But if you have any of the material start making the condenser.. 

First make a makeshift bowl using the materails on board. Now make another one but smaller than the previous one. Put the smaller one into the bigger one. Now pour some sea water in the large bowl.. Be cautios about not spilling any in the smaller bowll. fill it up as much as you can and wrap the open side of the bigger bowl with the metal sheet or the plastic sheet. Put something heavy and small on top centre of the sheet.it will help create condensation on the inside of the sheet and make it drop towards the smaller blank bowl. Put the whole set up directly under the sun. Wait until the cup fills and drink your salt free water.

Second method is use the pipe as a catheter and insert ot into your anal shaft. Yes, you read that right. put it into your anal shaft and pour water into your anal cavity.. That part of your body absorbs the water when dehydrated but leaves any substance dissolved in the water behind. But it is limited. You can do this once or twice a day before you become sick.

Now to answer your second question you can survive about 100-120 hours without water in a low temperature day. But as the temperature increases the more your body secrets water.. On thos kind of day you will survive for 75 and a few more added or reducted amount of  hours.. Hope this helps you in any way..