The Island

After asking any additional questions you are escorted back out and show to the smithy. The heat from the magical foirge spreads warmth throughout the smithy. Yet somehow you still feel a slight chill deep within. Nevertheless your talk with the commander calmed your hearts and worries enough to let you rest, though it is agreed that keeping a watch is a good idea "just to stay in practice". Early the next morning you wake, eat and set out.

Where to?

A ritual of this nature that keeps changing would require someone to constantly change and/or renew it's energies. A small temporary site could be created each time the spell was cast allowing it to move around the cemetery. However when the clergy and wizard from the base camp came to disrupt it they found no signs of this in the cemetery. This leaves the alternative, a larger more complex ritual that can be changed as needed. It is likely underground somewhere but they have not found any entrances to underground facilities in the cemetery. The fortress has some but it has been searched thoroughly, both by the guardians as well as the cleric and wizard from your camp, declaring it free of any outside influence at this time.
The commander suggests looking for any hidden passageways underground and any structure large enough to house stairs as a good point to start your search.

From what you saw of the cemetery there are few structures out of the water that might possibly have room inside, all being single room mausoleums. A couple of the structures in the river are much larger and even have multiple stories. These would definitely be able to hold a full ritual circle.

The river has obviously worn away at the ground around the submerged structures but only slants gradually. You could wade out quite far, dozens of yards at least, to the north and east. The slope is steeper to the west, towards the swift currents in the rivers middle. Most stretched on the western edge on the submerged cemetery are in ruins as the river citing wages them away.