Years ago we had serious fuel contamination problems due to underground steel storage tanks that rusted and leaked. Today all tanks are fiberglass, so this no longer happens (that I know of). However, those underground tanks do have fill plates on the ground surface (usually the parking lot) that can leak just like your boat deck plate. As the marina pumps its tanks nearly dry before the next fuel delivery, those who buy fuel from the near empty tank are the ones that are going to get the water (because it's pumped from the bottom of the tank). This despite the fact that the dock fuel pump has a water separating filter. I've opened the panels on occasion and have found the sight bowls completely filled with water, so at this point the water is being passed on to the customer. Next time you buy diesel, ask to see the filter at the pump! You have to remove the lower pump panel to see it.
However, it is important to note if you're getting water from your fuel retailer, chances are that it won't be a small amount. Most likely it will be a lot and your filters will fill up and engines crap out post-haste.
Just thought I'd comment on this bit and try to keep it non-technical (although I really should get some work done!):
Underground fuel storage tanks at marinas and service stations are typically double-skinned fibreglass; although aboveground ones can be fibreglass or steel.
When installed, the tanks are not level - one end is deliberately
lower than the other. Fuel is either pumped from just below the surface (called "floating suction") or from near the bottom of the tank at the high end. At the low end is a sump with a sample drain. Over here (Down Under), for the Big 4 Oil Co's, the water drains are checked daily
and any water pumped out - dunno what the regime is over there, but I assume it's the same.
The fuel bowser is fitted with a water-separating filter designed so that any entrained water is trapped at the bottom whilst the fuel flows out the side. The sight glass on these things shows the amount of water separated out of the fuel since last emptied. Some have an automatic cistern-style dump-valve that automatically drains the water away to a small holding tank when the sight-glass is full. Remember: The sight-glass is not
an indication that water is being pumped in to your tank!
Pascoe is correct that if you get water from your fuel retailer, it won't be a small amount. If you're worried, check your tank each time you fill up and before you start the engine - but I'd think that the retailer would be more worried than you are to allow this to happen - he could get sued! - and would make sure his sample points are checked regularly.
Hope you find this helpful..