GFCI outlets - can they be repaired? Are they really needed at the end of a circuit?
Not worth the risk
GFCI (whether in receptacles or breakers) are sensitive, life safety electronic items. As such, you don't want to mess with them. In fact, the whole purpose of the TEST/RESET buttons is because they can fail, even without obvious problems such as a flood. Newer models even test automatically.
Is it possible that there is simply still water inside the GFCIs so that disassemble, dry, reassemble will work? Yes. Is it a good idea to try that? ABSOLUTELY NOT!
New basic GFCI receptacles are in the $15 - $20 range (varies depending on brand, 15A vs. 20A and other features). Not as cheap as plain receptacles, but not that big in the grand scheme of things.
Presumably the GFCI receptacles were installed because the particular locations - basement, bathroom, laundry room, etc. required them. Assuming that is the case, replacement with plain receptacles would be a code violation. In most places you are not required to upgrade to GFCI unless you are making changes, but downgrading doesn't make sense and is likely a code violation.
Panel Alternative
An alternative is to install GFCI breakers in the main panel. This has the disadvantage that your panel may not have room for GFCI, or if you have a very old panel or fuses instead of breakers then that is not even an option. These breakers also typically cost more than &quot;ordinary breaker&quot; + &quot;GFCI receptacle&quot; so this change would be for practical reasons (e.g., protect outside circuits where a GFCI receptacle would be subject to severe weather) or as part of a panel upgrade. If you are looking at simple cost, replacing the GFCI receptacles where they are currently located will almost certainly be the way to go.
Check Out Everything Else
As Harper noted, there are plenty of other concerns as well. This is one situation where DIY may really not be enough. You could have hidden damage that can come back to burn you (literally) when you least expect it. Professional evaluation of the entire electrical system that was underwater is worthwhile.