Sven,
If your self-bailers are in the sump just ahead of the outboard transom, then it's best to keep one of them open when moored. Especially if you lack a cockpit cover to keep the rain out.
As an experiment, open one bailer and then get out of the boat onto the marina pontoon. The sump will slightly fill with water (maybe a few cm) and then stop. That is the self-regulating level beyond which it will not go, even if you pour many bucketsful of water into the cockpit. The excess will simply drain away through the bailer by gravity. There are three important provisos to this: if you keep the water ballast in, the sump will fill to the top but the water probably won't reach over the cockpit sole (until you get back into the boat); make sure that any bungs from the ballast tank into the sump are sealed tight, otherwise the ballast tank will slowly fill up from the sump; and whatever you do, don't weight the boat down by the bow, otherwise rain will collect at the front of the cockpit and not drain away.
One slight downside of this is that the bottom of your sump may get slimy to the top of the maximum water level. I even found elvers (baby eels) in my sump once.