undead boatman guy he just says something about needing payment
Have you been to Gno yet? I once read, that if you do things in the wrong order (i.e. talk to the boatsman to early) this bug will prevent you from going on. So maybe the safest order to do things would be
1. Get the Ice Orb
2. Talk to the Professors in Gno
3. Get the Coin
4. Talk to the boatsman
Same thing happens to me. I have the cursed doubloon, but when I talk to the boatman he says he's busy and I can't talk to him again after that.
I have the ice orb, talked to the necessary Professor at Gno, got the key to Upper Rain, went to the Rainbow's End Tavern and got the Flaming Sword, the hint for the bell to open the gate to Lower Rain, the 4th party (the big guy), and the cursed doubloon, then opened the gate to Lower Rain, killed the Werewolf, found the Boatman, and I can't get any further than that.