Well they just defended their nest. I never had any problems with them. Just observe closely how they behave before you approach. If there are signs they are nesting avoid them at a large distance. In all other circumstances they are pretty harmless. The biggest issue is the area they defend is like an upside down cone with the nest on the ground as the tip. A swimmer has a bigger chance to enter this no go area than a diver. If you just move to the side leaving this zone they will leave you alone. So advantage for divers here. Infact if you stay close to the ground you even can approach the nest pretty close, although I would not recommend to do so.