Did not see your entry until today, thus the late response.
I cannot give you diving advice there but I can make some sightseeing suggestions.
KoKee state park has some very nice views and its a great drive up to it. If you are really lucky, the clouds will be gone and you can see the coast. Many travel photo's from that spot but was always cloudy when I was there.
Kokee State Park link
There are some nice trails there. We hiked one that was on the ridge of Waimea Canyon. On your right when hiking, massive canyon that gets ~10" of rain per yr. On your left is rain forest that gets ~400" of rain /yr. People hike the first short portion to a waterfall.. you are on top...no getting too close to the edge as its ~2000 ft down. Few people hike further than the stream at the falls. We hiked for 6 hrs further and never saw a soul but be certain to give your self time to get back as the trail does not loop. At times the trail is narrow with a big drop on one side but the views are stunning and isolated. take all your food and water. A flashlight might be in order as its easy to hike too long. If you get to the 3 trail branch that includes Ditch trail, you have gone too far to get back before dark most likley. There is a gravel road on your left when facing the sign...you have to bushwack to find it as its about 100-200 yards into the forest. That is how we got back after hiking too long. If really really lucky, a local in a pickup will give you a ride back to your car, which is quite far by that time... it worked for us!
Sunset at the beach below is magnificent as you can watch the sunset into the ocean. Do not stay long after sunset if you want to find your way out. No signs out so we followed the few cars there that knew the way. When we were there, there were 17 of us. We each had our own mile, if we wanted it. Here is what Frommer's had to say about it.
Polihale State Park: This mini-Sahara on the western end of the island is Hawaii's biggest beach: 17 miles long and as wide as three football fields. This is a wonderful place to get away from it all, but don't forget your flip-flops -- the midday sand is hotter than a lava flow. The golden sands wrap around Kauai's northwestern shore from the Kekaha plantation town, just beyond Waimea, to where the ridgebacks of the Na Pali Coast begin. The state park includes ancient Hawaiian heiau (temple) and burial sites, a view of the "forbidden" island of Niihau, and the famed Barking Sands Beach, where footfalls sound like a barking dog. (Scientists say that the grains of sand are perforated with tiny echo chambers, which emit a "barking" sound when they rub together.)
Frommer's Kauai beach information
We followed small signs posted at intersections in sugar cane fields on the way to the furthest end of the park.
Another great hike is the Napali Coast trail.
Coastal hike info to a GREAT isolated beach
We hiked this trail and were glad we did. There were quite a few on the trail but the beach was worth it. No other way to get there as far as I could see.
We like hiking one day, water fun the next, etc.
