The North Shore of Oahu in winter is where they hold surfing contests
for professional surfers. Waves can easily reach in excess of 12 feet
on a moderate day. If you absolutely must kayak the North shore,
strongly suggest you speak with folks who paddle the 6 and 8 person
canoes as to where it might be possible. Seek native advice along
shore before attempting any kayaking. Personally I would only
remotely consider the North Shore in the summer after listening to the
marine forecast. I once asked a fellow who lived on the North Shore
about paddling it in February and his commment was, "Call SAR before
you go, wait fifteen minutes, and then paddle out to be rescued."

Lahaina channel is only safe for paddling early in the morning due to
an off-shore wind that can easily reach 20 knots by 11AM. The wind
combined with the current can be quite a problem. It is not uncommon
to have weekly "lost" kayakers in this area being rescued. In the
water by 6AM or 7AM and out by 10AM at the very latest. There also
tends to be a great deal of "gawking at the whales" boat traffic. Not
picking on the whale watching boats as they are usually the ones who
rescue the kayakers being blown out towards Tahiti.

Recently enacted law in Hawaii requires a VHF radio be carried if you
are more than a mile off the shoreline. This applies to kayakers.
When my wife and I kayak off the Big Island we always have a VHF along
with other safety equipment. Listen to the marine reports, talk to
the locals who go out daily.

--
Big Island Bob