You can reach Jachthaven Boei 12 by boat through the Slochterdiep and the Afwateringskanaal from Groningen. The Slochterdiep has bridges that you’ll need the provincial recreation key for to operate. You’ll also pass fixed bridges so you’ll have to lower your mast.
From the Eemskanaal near Appingedam you’ll pass the lock to the Groeve, then you’ll pass the bridge to Overschild (you operate this yourself with the provincial recreation key) and then you’ll arrive at Schildmeer Lake. This approach has a sailing depth of 1.5 metres and a maximum erect mast height of 16 metres.
Schildmeer Lake is connected to the Eemskanaal and from there you can easily sail to Friesland via Groningen or sail towards Delfzijl and then get onto the Eems or the Dollard.