Blair, you always have such interesting questions. One thing you may want to consider, now that you know your boat a tad better, is to reread the C36 Manual. I find doing that, for me even after 14 years of ownership, helps a lot. Never has hurt.

Blair,

The fuel tank vent is on the upper port side of the transom, there have been some conversations about that here on the forum. There is no vent valve and it can spit a fair amount of fuel out the vent before you realize this.
This can be an issue as you can be fined if noted by the wrong person.

As for your water tanks, you should not have had any overflow into the boat. The vent for the water tank is on the starboard side of the cockpit in front of the helm seat. If you had water in your bilge it is likely that this vent line has ruptured or is disconnected. You must use some caution as if you put a water hose with a tight fitting nozzle into the fill port and have the pressure up on the hose, it will pressurze the tanks and vent system and you could burst something with the pressure as the vent is fairly small and may not vent fast enough for the water inflow rate. If I wait until the vent starts spitting out water before I pull out the hose, I will get a small gyser coming out of the fill port when the hose is removed from the port.

And yet again, Blair, I suggest doing a search on the forum for this much-discussed topic.

On my Mk II, I fuel with TWO limits in mind, stopping fueling at *whichever limit comes first*:

1) the sound of the fuel in the filler hose starts to rise in pitch;

2) the gurgling of the fuel tank vent ceases.

In a noisy environment, Step 2 sometimes requires another person to listen very carefully near the vent. In the meanwhile, the person listening at the vent can also be watching for fuel to spurt out of the vent and be ready with a diaper to capture it. In my boat, the #2 Limit above is the first indication of being full.

As in small airplanes, fuel gauges on our class of boat are notoriously inaccurate. I am contemplating putting a sight gauge in my boat's aft cabin to allow visual assessment of the fuel level in the tank. In a post here a number of years ago I suggested this idea, saying that a flashlight glance at the fuel level would be very helpful. Steve Frost replied that, as some of our boats had aluminum tanks, that a quite powerful flashlight would be needed.

Marine stores self a small plastic container with suction cups that fits over the vent so any fuel that gets kicked out thru the vent is caught. You may want to try that.

In any case, I have that device but I just have my wife watch the fuel guage (with the key "on") as I fuel and I go REALLY slow when she tells me the needle is hitting "full".

And my solution - I cut a one quart plastic container of auto oil in half along the vertical height and drilled a hole on one side large enough to fit snugly over the circular fuel vent hole. The hole is at the upper end of the container. Then before fueling I hang the container over the vent and place some absorbent cloth inside. My wife listens to the fuel sound and when it becomes louder I quit fueling due to the rising level of her voice. If any fuel is to spill, it is contained in this container. I discard the pad if this happens. I only spilled the first time I fueled. This has not happened yet. I am ready if it is going to happen.