Seems quite odd that the coast guard would have a higher duty of care than a school or a sheriff's office. Has there been any successful lawsuits where the coast guard failed to save someone and were sued?
I found this[0] but it appears to be ongoing. The state appears to again argue they have no duty to rescue anyone.
This case is a good example of the limits -- it's been held that once USCG begins an operation, they fall under "good Samaritan" requirements, which means (roughly) that they can't make things worse; and, if things get worse, a FTCA suit could force them to demonstrate they followed through without negligent acts or omission based on their existing practices, with due deference to the right USCG has to make reasonable discretionary decisions. (I don't have any case cites at hand, but IIRC USCG has lost a couple after calling off searches early or issuing confusing notices to private vessels also engaged in searches.)
Whether this would apply to the instant case is arguable, but turns mostly on the definition of when the duty of care kicks in (I think there's a decently-strong case to be made that once they stopped the "vessel," they pretty much didn't have an option to let it go). Interestingly, while SCOTUS has steadily limited government duty of care, this hasn't extended as much to conditions of public hazards where the police or government has specific knowledge of the risks: a cop doesn't have to charge into a school shooting situation to save your child, but there are conditions where, if they fail to correct a traffic hazard they had knowledge of and control over, you would have a case if you were injured. Hazards at sea are probably more similar to the latter case than the former.
It's worth noting that USCG doesn't have any statutory mandate to rescue anyone (their duties enumerated in 14 USC §102 say nothing about saving distressed mariners), so your fundamental point is correct: for example, if someone had called in "crazy guy in hamster wheel out at sea," USCG could likely decide "not my sea monkey, not my circus" and let it go; it's just that once they're on-site it's a more legally-fraught situation. (This is on top of the fact that the Coast Guard treats rescue as a core mandate, so no chance they weren't going to stop this guy from ending up Gulf Stream flotsam.)
I found this[0] but it appears to be ongoing. The state appears to again argue they have no duty to rescue anyone.
[0]https://www.courthousenews.com/lawsuit-over-coast-guard-call...