Depending on where you fill up, fuel quality is often dubious. Roadside petrol stations have mandatory quality monitoring in the EU, with compliance over 90% except in Greece where it's under 90% compliance. Marine fuel stations are much worse - less monitoring (if any), proximity to water, irregular use (often the fuel sits all winter in a poorly maintained tank) and operator errors. Unlike fuel stations, they're also often independent and thus try to save money by buying cheap fuel from dubious sources themselves.
Fuel delivery trucks (common in small Greek harbours) are totally random - look for ones operated by a big oil company (truck painted in livery, not just a logo stuck on), small independent suppliers are more prone to dodgy fuel. You also sometimes get cheated on the meter
Some people pour all their fuel through a filter with water separator, but as that takes ages, it's usually frowned upon at most refilling docks. It's fine if you fill into cans first and then pour it in the tank on your own time though.
We mostly haul jerrycans because roadside fuel is 30 cent less per litre in Sicily and of reliable quality - and there's not many fuel docks anyways. In Greece we ordered the occasional tanker and learnt a lot about them (like when the guy claims you bought more fuel than fits in your tank, or drags an old rubber hose over your deck, leaving black marks everywhere).
For maintenance, I pump up the dregs from the bottom of the tank by way of a copper tube stuffed down one of the spare fuel take-offs on the tank. Gets in the corners and slurps up the dirt and water. Oh, and the O-Ring on the Roca filler cap gets the occasional bit of silicone grease and replaced when it goes stiff.
We've only had some dirt and never any water since doing the above, but it only takes one dodgy fuel dock or trying to fill up while it's raining
