There is only one Dopefish in the final segment. To get past him, hug the far wall of the well, and let him grab a dopefish. This is a bit safer than trying the method used on Normal mode, where you stop and let him eat a schoolfish.
On normal level, there are two Dopefish at the final segment. You can't hug the wall anymore, as the second one will eat you while the first is burping. So, to get past, you'll need some schoolfish following you, ideally a bit spread out. Go fast down towards the first dopefish: pass under it, giving it a schoolfish. Then, hug the bottom of the well, and surface up - you can move faster than the dopefish in a straight line, and it will likely snarf another. You need enough space so that when you rise directly up through the main well you have your schoolfish behind you. When you stop, they will keep going - you'll need to have the second Dopefish snarf one, or he'll get you. While he's burping, you can make your escape from both Dopefish and grab the Wizard.
When you start, don't go straight and through the door; there's some flying bugs on the other side. Just go up the pole.