Welcome to the forum.
You should have a bleeder on the bottom of the filter. If you don't have a tube attached to the bleeder, I would suggest running a hose or tube to someplace easy/convenient and occasionally (like right now due to the Water In Fuel light being on) pulling off about one clear beer bottle (you could use a clear jar, but I always have a beer bottle handy) of fuel and taking a look at it. If your truck sat, or you got water/algae/rust/any other foreign matter in your fuel tank, you'll get to take a look at it.
You should also start paying attention to where you are buying fuel. I lived in the Real Washington, and I got a tank of fuel that had one full beer bottle of water in it. Shut my truck down like the turning the key off. It's hard to prove where/how it got in there, but I never bought fuel from that location ever again.
If you happen to change your fuel filter during this WIF investigation, do a quick search and follow the directions (engine hot and AFT or dieselkleen in the filter, don't crank the starter too much).
Good luck and again welcome to the forum.