No passengers were on board when the vessel from the MS Voyager that by the pontoon connected to the Albert Pier. It was refloated at 13:50 BST.
A harbour spokesman said the tender's crew had been advised earlier to move to a deeper area next to the pontoon.
Permanent pontoons, suitable for all tides, are due to be completed by May.
It follows a move of the drop off/pick up point for cruise tenders from the inter-island quay to the Albert Pier.
Harbour master captain Chad Murray said one of the key challenges at St Peter Port was the large number of areas that dry out at low water.
He said there had been previous incidents of grounding tenders at the inter-island quay.
A spokesman for the Public Services Department said the three new pontoons would allow four tenders to dock at one time with passengers able to walk ashore at anytime.
He said on days when cruise liners are not visiting the pontoons will be available to the wider boating community and the pontoons were being funded through visiting cruise liner fees.
A tender from a cruise liner visiting Guernsey was aground for an hour as it tried to pick up passengers using a temporary pontoon.