FKK Sharks actually has the most convenient public transportation!
I visited FKK Sharks twice on this vacation.
At the first time, I took the train to Darmstadt Nord Bahnhof, and then walked to FKK Sharks.
At the second time, I took the train to Darmstadt Haupbahnhof, and then took the "Bus R" to FKK Sharks.
I found that FKK Sharks actually has the most convenient public transportation among Hessen Big Five!
There is a bus stop called "Daimlerweg" just literally outside the club.
From the bus stop "Daimlerweg", it took me less than 3 minutes to walk into the FKK Sharks entrance.
I later found that I can take the "Bus R" from both Darmstadt Hauptbahnhof and Darmstadt Nord Bahnhof to "Daimlerweg".
The "Bus R" route is like this: Darmstadt Hauptbahnhof-Daimlerweg-Darmstadt Nord Bahnhof.
So, one can easily reach FKK Sharks simply via "Train + Bus R + a very short 3-minute walk".
Of course, if one is under time pressure (if you want to catch the last train back to Frankfurt) , one should just take a taxi.
On my way back, I took the taxi from FKK Sharks to Darmstadt Hauptbahnhof twice.
The drivers would use the meter, and the price from FKK Sharks to Darmstadt Hauptbahnhof was 8 EUR-9 EUR.
However, you should always be prepared to spend one piece of ten banknote for the taxi fare, because the drivers may not have the change for you.
(I encounter this situation twice. The price on the meter was 8 EUR-9 EUR, so I gave the driver a ten banknote, but the driver said he had no change. So, I just told the driver to keep the change and got out of the taxi. LOL).