Arrangement for interconnecting concrete pontoons for small boat harbors, fish raising plants and the like, are subjected to very severe stresses as a result of wave movements and the like. In order to obtain a more safe and reliable interconnecting arrangement at least one side or end of each pontoon (1) has two bumper surfaces (11AF, 12AF) spaced from each other and adapted to face against and cooperate with corresponding two bumper surfaces (11BF, 12BF) on another pontoon (2). Between the respective bumper surfaces on two interconnected pontoons there is mounted a fender (11, 12) which is compressed with a significant degree of prestressing. One pontoon (1) has two symmetric and laterially located attachment members (17, 18) each for .ang. separate tension element (13, 14). The other pontoon (2) has an attachment member (15) centrally between its bumper surfaces (11BF, 12BF) for the common anchoring of both tension elements (13, 14). There is provided for a certain degree of articulate or rotary motion in the anchoring of the tension elements (13, 14) to the attachment members (15, 17, 18). In principle the arrangement may be considered to function as a universal joint.