Possible leak near skimmer causing corrosion

I have a >25yr-old, 30,000+ gal., inground, gunite pool that came with the house I bought four years ago. Over that time I focused on the pool equipment out of necessity. Now, I have recently started losing 0.25" of water per day when I previously lost only 1" per week at the same time of year.

In an effort to identify the leak, I allowed the pool level to fall until a baseline could be established. Within a 0.25" of the bottom ledge of the skimmer opening, the water loss slowed to the seaonal norm.

Coupled with this, I have found flakes of corroded metal (presumably steel) in the opening to the skimmer basin that has dropped from the steel plate (that supports the edging) above along with rust staining of tile leading to the skimmer basin.

Although I cannot find any visible cracks under the water line in the skimmer, I cannot assess the integrity of the bond between the skimmer basin and the concrete either. Without tediously leak testing the rest of the pool, I am wondering whether I should try addressing this problem area first, applying Occam's razor, before trying to detect a leak elsewhere in the pool along the water line (~120 linear ft).

If so, what kind of sealant can seal joints between the basin and the concrete? Some net postings suggest marine caulk, aquarium sealant, poly/silicone caulk. I am especially concerned about the adhesion, durability and mildew resistance. And secondarily, the cure time.

I would prefer not to have to replace the steel member, just eliminate further corrosion.

Any thoughts?