The biggest difference between summer and winter in Hawaii are the state ocean and direction of the wind, not the temperature. In the winter the north and west shores get hit with waves generated by winter storms far out to sea in the North Pacific, these waves are famous for surfing. Even if the surf is not extremely high, the winter conditions churn up the sand and make visibility poor for snorkeling. Although it is always possible to get lucky and have a truly calm day on the north shore in winter, they are rare. (North Shore beaches are also much more dangerous in winter because the high surf triggers rip currents. see this
recent post about Beach Safety)
You would think then that south shore beaches would be OK but in winter the NE tradewinds give way to SW Kona winds and the south shore waters gets churned up from waves generated by on-shore winds.
Some snorkel beaches have protective reefs that reduce the wave height but the wave action still churns up the sand, reducing visibility. Even well protected beaches like Salt Pond Park on the south shore get churned up in Kona winds. A reefed area like Tunnels Beach is even more dangerous than usual in high surf even though the reef appears to be knocking down the waves. The water that overflows the reef has to get out of the lagoon somehow and it flows out as a rip current through gaps in the reef. If you are snorkeling along the reef and hit the gap, you are gone! You can't swim back through the gap against the current and you can't come over the reef or you will be torn to shreds.
We have been to Kauai in winter 3 times and summer once during the past 5 years and have never had a good, clear snorkeling day in winter but on our one summer trip the snorkeling was fine almost every time we went out, either on the Big Island or Kauai.
