Storm protection barriers have been damaged by strong winds combined with very high tides.
The support piling and the pontoon wall at St Sampson's Harbour were damaged overnight.
Guernsey Harbours' maintenance staff are carrying out repairs, where the tidal conditions allow.
A spokesman urged berth holders to check their mooring ropes during bad weather and advised mariners "to exercise caution in the area".
A tide warning has been issued for Tuesday with a 10m tide at 19:22 GMT alongside force 5-7 easterly winds.