This may sound like a dumb question, but ... the article says they hope to find the spring. If it is under water, unless they know where it is, how do they know it was there ... or, anywhere?
It’s a SpongeBob thing.
They’re not looking for a specific spring. They are looking for a spring as in “any spring,” though probably they would hope to find a large one because it would offer the most hope of success in finding archaeologically significant site. They can follow an old riverbed because the erosive effects of running water leave evidence. If you follow any Forida watercourse you’re going to run onto a spring sooner or later, probably several, perhaps even large sinkholes even if you extend the search out along its old bed, now submerged in the sea.
Freshwater springs continue to produce even when submerged in the ocean. There is also a temperature difference between the the emerging water and its saltier surroundings though I don’t know how detectible this would be at great depths.
They can also look at the karst topography on land and try to follow any trending fractures or linear features in hope of finding the remains of a sinkhole.