The body can not survive for longer than 3 or 4 days without water, so your first priority should be to find water to stay hydrated.
Sources of Drinking Water:

Recycled Water (Urine):
There have been stories of survivors using urine as a last resort to replenishing the body. The truth is that many survival instructors (as well as the US Army Field Manual) advise against drinking urine as a means of hydrating the body. The salts in urine will worsen dehydration and make you even more thirsty.

Rain Water:
If it rains, try to set out as much material as possible to collect it and funnel it into containers.
Before bottling rain water that has fallen into the raft, make sure it hasn’t mixed with ocean salt water that may have also splashed into the raft.