I generally feel 15MPH is an upper limit. Altho this is a great and reasonably sheltered place to practice wave handling. Several islands occur in PH with nice pullouts for a picnic lunch. There are some currents in the bay during tidal flushes and currents under bridges can be strong but manageable. The bridge between PH and Little Harbor near the Wentworth Yacht Club can produce very strong currents during most of the tide cycle.
Caution is required and a fast paddle stroke to get through against the current.
Little Harbor opens up onto the Atlantic but is sheltered from the waves by a breakwater. Past the breakwater one finds the Isl of Shoals 7 nm away - next stop Europe. This is surf country for those so inclined. Portsmouth has been busy this spring, numerous kayaks on the water. New place to launch must be found-too bad PI was a great launch.