These are two different projects that sometimes get confused, and picking the right one matters for both cost and outcome.
Pond excavation
This is digging a new pond from scratch — evaluating a site, planning size and depth, and excavating a pond where one doesn't currently exist. It's a larger project in terms of planning since there's no existing structure to build from.
Pond rehabilitation
This covers restoring an existing pond — removing silt, repairing banks, fixing a spillway, or in some cases enlarging the pond's footprint or depth. It generally costs less than a new excavation because much of the groundwork already exists.
The exception is a pond with fundamental design problems. If a pond was built without enough watershed to stay full, or in a spot where it was never going to hold water reliably, rehabilitation can only do so much. In that case, a fresh site evaluation is worth doing before spending money on a repeat of the same problem.