I've already posted my tale of woe with diesel but it might bear repeating, I have a large fuel tank amidships with awkward access, the tank has a disc cut out of it to take various pipes, its about 6" dia but I cut it by drilling a ring of holes, to put your arm inside is like tickling the tonsils of a crocodile. When the engine stopped a couple of years ago I did the filter and watertrap to no avail, the fuel smelt wrong so eventually undid the ring of bolts and looked into the tank. Horrible orange mess, suspect the deck filler had leaked but the only way to clean it was to buy two plastic barrels, put a bilge pump in the tank and empty the lot. The mess that was left had to be cleaned with rags and nappies (careful with those, they disintegrate and the gel comes out), it took days to do it properly, my arm was shredded from the inspection hole and the far corners of the tank took ages to clean with rags and sponges on sticks. Having done all that I am absolutely sure that sucking the bottom layer out would simply not have worked, the slimy mess all over the bottom would have not been touched. The 'fuel' that came out was all scrap. If I had another boat I would specify a proper inspection plate be fitted. I now remove the plate every year and check it. I would recommend poking a little camera into the tank if you can't get decent access especially if you venture onto rivers. Water in fuel is a real sod.