Certainly sounds like a blocked vent
But why are you asking here instead of investigating for yourself? You obviously know the symptoms of blocked vent--tank is pressurized and you can't pump out. Did you inspect the vent thru-hull (the most likely place for a clog in a vent to occur) to see if it's clogged? Did you try backflushing it with a hose? If not, why not? I don't mean to be unkind, but at sea there are no plumbers or electricians or mechanics, and internet access to ask questions you could answer for yourself on a forum can be hard to come by 1000 miles from shore...and it's not unreasonable that you would want to sail a 35' boat to Bermuda. That makes it absolutely essential for boat owners--sailboat owners especially--to know every inch of their boats, learn to trouble-shoot for themselves and try to cure what they can--to take as much responsbility for your own boat as possible. So if you're a new boat owner, asking how to clear a blocked vent is one thing...but why don't you even know for certain whether it is blocked--especially since you know the symptoms? This time it's "only" a blocked holding tank vent--a major PITA, but not life-threatening...but next time it could be something that puts you, your boat and your crew at risk, even only a few miles from shore. So I'm not just beating you up for the fun of it, I'm trying to teach you how to save your life some day! Now go check that vent line, and if you can't figure out how to clear it, I'll be glad to explain it. And while you're at it, check ALL your vent through thru-hulls...it should be a matter of routine maintenance to check ALL vent thru-hulls regularly for signs of blockage...every time you wash the boat is a good time to do it. And the holding tank vent should be backflushed with a hose every time you pump out to keep it clear.
