Bottom line up front: I had an emergency and discovered a large amount of water in the tank post-flight. I'm happy to be here telling you this and asking the POA mind about this event... Went to go fly Saturday for the first time in a few weeks. The plane is currently sitting on a ramp as I wait for my hangar to be ready to move into (in a couple weeks). When I got to the plane I did my normal pre-flight, including sumping the tanks, checking for water/contamination, and finding nothing but Avgas, pouring the sample back into the tank while also visually checking the tanks were indeed topped off. Taxied, did my run-up, took off, worked with ATC to get on FF and transit a Bravo, and got to my cruise altitude of 6,500' before switching tanks for the first time. A minute or two later, I lost engine power. I was with ATC, got vectors to the nearest field, went through my checklist, gained power again at mid-RPM, pointed the nose to a large towered field, stayed high enough until I had the landing made, dropped flaps/slipped it in and landed uneventfully. The trucks greeted me, the engine was shut down, had the plane towed to a maintenance hangar. We open the fuel drain, and nothing but water came out for what seemed like 10 minutes (was probably 10 seconds, but that's when I realized how glad I was to be here today). I can go into more details about the emergency if people are curious, but I wanted to share a couple thoughts and ask a couple questions. Background/Pre-Flight: *B1: I've never had water in my tanks in the 2 years I've owned the plane. After the last annual there was some expected residue due to fuel lines being replaced, but I sumped it out until nothing but Avgas was being caught. *B2: Last flew a few weeks ago, plane is on ramp outside and had been rained on plenty. Some of it very heavy rain. *B3: This past week I had both wing fuel drains replaced since the last few flights the gas was slow to drain, and I assumed the o-rings were dry or something was starting to clog the drains. It's cheap and easy and fixed the issue since the gas poured out of the drains during pre-flight. *B4: During pre-flight, everything looked normal when sumping. I had the blue tint, the smell, no signs of water or contamination. *B5: Also during pre-flight I did not notice that a cap was loose after it had been refueled. Flight: *F1: No issues during run-up or the first 20 minutes of flight. *F2: Got to cruise altitude and swapped tanks, and still no issue for a couple minutes before power loss. *F3: Throttle felt "loose" as if it were a mechanical failure of a linkage. *F4: Got power again mid-RPM and I used that to get to a preferred larger airfield/runway. Post-Flight: *P1: Engine died/shut down after taxi off the runway at idle power (I did not shut the engine down myself). *P2: When we sumped the engine drain, it was just water. Scariest thing I've ever seen. *P3: Mechanic today is going to drain everything, check the carburetor/anywhere else water could have gotten in or settled. Thoughts: *T1: I was remarkably calm during these events. I had always wondered how I would react, but I just knew I had to fly the plane. It's what you train for. I was on FF already and that communication helped reassure/remind me to just fly the plane. *T2: I ALMOST ran the entire emergency checklist. I failed to swap BACK to the original tank. I'm still kicking myself for this 48 hours later because how did I miss this step, especially when it would have sent pure gas back to the engine and I would have landed, probably with full power/control and a better understanding of what was happening in flight. *T3: I am going to spend several flights walking through emergency checklists in my future. I usually practice these things, but not often enough. I'm sure many of us fall into that category. *T4: I'm going to say the steps of the checklist out loud and run the list at least a second time (if time allows) so that I don't miss a step that could save my life. *T5: I'm always going to take my time after sumping to not just glance at it and pour it back in. I feel like I always make sure it's gas, but now I'm questioning if I really (no, really) look and smell to make sure. Questions: *Q1: Could water have gotten into the tank during maintenance when the drains were replaced? If so, could it have gotten trapped and not fallen to the bottom? *Q2: If the tank cap/gasket was allowing rain water in, could any of it have gotten trapped and not fallen to the bottom of the tank? *Q3: If the plane sits outside and gets rained on regularly right now, but no water had ever been found in the tank, could a cap/gasket suddenly go bad or would it have started to allow a little bit of water in? *Q4: If the cap had not been fully closed after the last refuel (which was done by the FBO after my last flight), would I have noticed during pre-flight that it didn't "feel" right or "unlock" when I opened the cap? *Q5: Could I have missed it if I sumped ONLY water from a wing tank? I'm sure if I caught water AND fuel and saw the separation it would have been impossible to miss. But what about nothing but water and I thought it was blue and missed the lack of a fuel smell? *Q6: If I missed it during pre-flight (didn't miss the step, but missed that it was pure water), how do I never do that again?? So these are the possibilities I can think of. Either water was stuck and didn't drain when I did my pre-flight (how?) OR it did drain when I did my pre-flight and I somehow missed it (how?). Please let me know your thoughts. I want to make sure I'm not missing something and I have my head around this situation. Asked another way, how do I confidently assure my wife that this will never happen again? EDIT Forgot to add the aircraft type: P28A-181. 1976 Piper Archer II.