Stephan, sounds like you have the right system for your cruising grounds but the wrong system for your marina, and even IF the marina says it's OK, your neighbors aren't "smart" enough to know it is OK.You get that we don't have a holding tank, right? Essentially we can't use the toilet and so can't stay at that Marina if no discharge is allowed.
That's your BIG issue: PERCEPTION. Even if it is legal.
THAT"S what you need to deal with.
You then asked how big the holding tank would need to be. Is it possible to do the math? I'm pretty sure the instruction manual for your system says exactly how much water is used for each flush...