There wasn't an organised excursion here but you could use the town shuttle which used a circular route, stop 1 at the town centre, and stop 2 at the Keroman base.  The thumbnail map links to the shuttle bus route which includes the submarine base in the bottom left hand corner,  But this link gives just the three main buildings; K1, K2 and K3 with the French submarine Flore S645 between K1 and K2.  There was a guided tour of the site in English, but I only went to one of the museums (the one in K1) which includes the submarine.  As far as I can make out, only K3 was used for garaging U-boats during reprovisioning, the same as the other pens at La Rochelle, Bordeaux and Bergen.   K1 had no access from the water and K2 had only 1 pen that could be entered from the harbour. What seems to have happened is that the U-boats were brought in to K2, then loaded on trolleys and moved to the other pens on rails.  I believe that K1 and K2 formed the major repair facility for all the U-boats of the fleet, while minor repairs could be done in any of the pens.  The allies were unable to inflict major damage on the facilty, so implemented a scorched earth policy of flattening the town by bombing it to deny resupply of the base.   This effectively removed the U-boats trapped there from the war although the German army defended the base until the end.

There are two U-boat museums here, although I only visited one, the second is in K2.  There is also a museum of sail and a yachting marina plus some childrens amusements.   So the site is worth a visit.