Something to keep in mind when fueling on land: the boat will be sitting reasonably level on the trailer. It may not sit the same once in the water, AND there will be a slope as you back down the ramp.
When I drove the cruise boats in the Tetons, the company also ran the marina. One day, an older boat backed down the ramp (a half mile or so from the marina office), and the slope started a siphon that made gas pour out of the vent. By the time someone came running into the office to report it, that boat/trailer/truck had pulled out in a hurry and disappeared. Left us with a real mess to clean up. In a pristine National Park.
We had trained for this, and within 15 minutes had it contained with booms and were using absorbent pads to get the fuel out of the water. A few minutes later, the NPS folks showed up and demanded to know what happened... "Kinda busy here, guys, but it wasn't our doing." They wanted someone to blame for it - I will tell you that they take this very serious.
They wanted to know if we had put any soap in the water... nope. That will make the sheen disappear, but just pulls the fuel more into the water, rather than sit on top of it.
Some marina fuel docks are, likewise, serious about any spill into the water. We made it a point to open both fuel caps on the 25, listen for the sound in the fuel line to change, and kept a diaper (absorbent pad) at the fuel vent when fueling. And, not top the fuel completely up.
The larger marinas often have high and low pressure pumps for diesel, just one for gas. Go easy.