Anti radiation meds.
also you've got 2 minutes from when you get the emission message + a fair whack of extra time during it (it only kills you at the very end) you can sprint all the way back to the boat with the trader and stuff in it from there in plenty of time.
I pretty much only just started, i'm being a STALKER noob. Wandered to the outpost, got the quest, wandered to the boat, went outside, got shot up, emission warning, slowly die no matter what i do. Maybe i should just start again and pay more attention