Disconnect the hoses on the "feed" side of each pump. I use a small piece of hose and attach one side to the incoming/feed side of the pump then jam a funnel in on the other side. Turn the pump on and pour in some non-tox/antifreeze down the funnel until you see it come out.
If you dont have any strainers in between the pump and seacock, you can usually get away without dumping any antifreeze down the disconnected hose, as it will usually drain out by itself.
If you want to play it safe, or have a strainer pour some antifreeze down the disconnected hose until you see it come out the bottom of the boat, or wherever the pickup is. This way you know you've gotten rid of all the salt/fresh water.