When I took the cover off the fuel tank to clean the fuel level sender this year, I looked into the tank for the first time since we got the boat 9 years ago. It had a few wisps of crud, but really not much to worry about. We do treat the fuel with a biocide. Not sure if that's entirely necessary.

Here's a photo of what we did. The implementation is original but the idea isn't ours.

Our tank is set up with a "sludge" pickup at the very bottom of the tank. After fueling or before a rocky/rolly passage, we use the permanently mounted pump to draw a fuel sample. If there's water or crud, it should be in that sample. The regular fuel pickup is a bit up from the bottom of the tank. If the sample is fine, I just pour it back in through the deck fill. Since sampling is easy to do, I actually check the fuel quality fairly often.

I cut in the clear deck plate so I can get quick access to the fuel shutoff valves.

We have dual oversized Racors with a selector valve. We couldn't use 500's, so use the spin-on type. We added a vacuum gauge on the engine side of those filters. If the final engine filter is getting plugged up, this vacuum gauge won't know about it, however.

So far, so good.